Method and apparatus for CO2 sequestration

ABSTRACT

Processes, methods, and apparatus for carbon sequestration utilizing catalysis schemes configured to provide high concentrations of hydrated CO 2  in proximity with a sequestration agent are provided. Reactants are combined with catalyst such that at least two regions of controlled catalytic activity form encompassing at least the interface between a sequestration agent and an aqueous solution containing dissolved CO 2 . Suitable reactants include various sequestration agents, catalyst, and carbon dioxide dissolved in an aqueous solution (seawater, for example). Possible products include bicarbonate and metal cations.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/975,584 filed Dec. 18, 2015, which application claims priority toU.S. Provisional Patent Application No. 62/093,958 filed Dec. 18, 2014and U.S. Provisional Patent Application No. 62/208,356 filed Aug. 21,2015, the disclosures of which are incorporated herein by reference.

STATEMENT OF FEDERAL FUNDING

This invention was made with government support under 1220600 awarded bythe National Science Foundation. The government has certain rights inthe invention.

FIELD OF THE INVENTION

The present disclosure is directed to methods and apparatus for CO₂sequestration.

BACKGROUND OF THE INVENTION

Carbon dioxide (CO₂) constitutes about 0.04% (400 parts per million) ofthe atmosphere. Despite its relatively small overall concentration, CO₂is a potent greenhouse gas that plays an important role in regulatingthe Earth's surface temperature. Presently, anthropogenic CO₂ generationis taking place at a rate greater than it is being consumed and/orstored, leading to increasing concentrations of CO₂ in the atmosphere.There is a growing concern that rising levels of CO₂ in the earth'satmosphere may present a substantial environmental challenge. As aresult, there is an increased interest in developing methods forremoving CO₂ from emission streams and the atmosphere and storing it ina manner that prevents its future release into the atmosphere. Thiscapture and storage is collectively known as CO₂ sequestration.

BRIEF SUMMARY OF THE INVENTION

The disclosure is generally directed to apparatus and methods for thecapture and sequestration of CO₂ via a novel catalytic system.

Many embodiments are directed to a method for carbon dioxidesequestration including:

-   -   dissolving carbon dioxide into an aqueous solution to form an        aqueous carbon dioxide solution defined by a mineral        undersaturation level;    -   combining the aqueous carbon dioxide solution with a        sequestration agent;    -   titrating a hydrating catalyst into the aqueous carbon dioxide        solution such that at least within a first catalysis region a        mixture of catalyst and aqueous carbon dioxide solution is        formed, said first catalysis region encompassing a second        interfacial catalysis region located within the laminar boundary        layer at the interface between the mixture and the carbonate        sequestration agent; and    -   reacting the aqueous carbon dioxide solution with the catalyst        within the first catalysis region to produce protons in        proximity to the second interfacial catalysis region such that        the protons dissolve the sequestration agent;    -   reacting the carbon dioxide within the aqueous carbon dioxide        solution with the dissolved sequestration agent within the        second interfacial catalysis region to produce an effluent        comprising at least bicarbonate; and    -   wherein within the second interfacial catalysis region the        dissolution of the sequestration agent is enhanced such that the        overall rate of dissolution of the sequestration agent within        the catalytic region is higher than the rate of the uncatalyzed        dissolution of the sequestration agent when exposed to an        aqueous carbon dioxide solution having the same mineral        undersaturation level.

In some other embodiments the sequestration agent is selected from thegroup consisting of a metal carbonate, or a silicate mineral.

In still other embodiments the sequestration agent is calcium carbonateand the catalyst is one of either carbonic anhydrase or a carbonicanhydrase analog.

In yet other embodiments the overall rate of dissolution of thecarbonate sequestration agent is at least an order of magnitude higherthan the rate of the uncatalyzed dissolution of the carbonatesequestration agent when exposed to an aqueous carbon dioxide solutionhaving the same mineral undersaturation level.

In still yet other embodiments the mineral undersaturation level is heldat less than 0.5.

In still yet other embodiments the method further includes placing atleast the first catalysis region and the second interfacial catalysisregion under a pressure of at least 500 psi such that the dissolution ofthe sequestration agent is increased relative to the unpressurizeddissolution rate of the sequestration agent at the same mineralundersaturation.

In still yet other embodiments the method includes maintaining at leastthe second interfacial catalysis region at a temperature no greater than200° C.

In still yet other embodiments further includes reacting with acondition agent the aqueous solution to reduce surface poisoning ions inthe aqueous carbon dioxide solution.

In still yet other embodiments the aqueous solution has a circum-neutralpH.

In still yet other embodiments the aqueous solution is a brine solution.

In still yet other embodiments the aqueous carbon dioxide solution iscombined in measured aliquots such that the mineral undersaturationlevel is maintained at a constant level.

In still yet other embodiments the method further includes stirring theaqueous solution within at least the first catalysis region such that amixing zone forms wherein the aqueous carbon dioxide solution andcatalyst intermingle and wherein the mixing zone is within the firstcatalysis region.

In still yet other embodiments the stirring forms a diffusion boundarylayer around the second interfacial catalysis region the diffusionboundary defining a volume around the interfacial region of thesequestration agent on the order of 10 microns.

In still yet other embodiments the method further includes rougheningthe surface of the sequestration agent such that the grain size of thesequestration agent is no greater than 100 μm.

In still yet other embodiments the method further includes collectingand filtering the effluent from the reaction to capture at least one ofcatalyst or unreacted aqueous carbon dioxide solution, and reintroducingthe catalyst and unreacted aqueous carbon dioxide solution into thefirst catalysis region.

In still yet other embodiments the catalyst operates to at leastcatalyze the protolysis of water in the aqueous solution and hydrate theCO₂ within the solution.

In still yet other embodiments the rate of dissolution is diffusion ratelimited.

In still yet other embodiments at least one of either the pressure isincreased or the temperature is decreased to increase mineralundersaturation.

Many other embodiments are directed to an apparatus for sequesteringcarbon dioxide, including:

-   -   at least one reactor vessel defining an enclosed volume;    -   at least one source of a catalyst, a sequestration agent, a CO₂        gas, and an aqueous solution;    -   at least one input in fluid communication between the at least        one source and the enclosed volume of the at least one reactor        vessel; and    -   at least one output in fluid communication with the enclosed        volume of the at least one reactor vessel;    -   wherein the at least one input is arranged such that the CO₂ gas        and aqueous solution combine to form an aqueous carbon dioxide        solution, and wherein the aqueous carbon dioxide solution and        catalyst are delivered as a mixture within the enclosed volume        of the at least one reactor within a first catalytic region        encompassing a second interfacial catalytic region disposed        about the sequestration agent and being located within a laminar        flow boundary at the interface between the mixture and the        carbonate sequestration agent.

In other embodiments at least one of the sequestration agent andcatalyst is physically confined within the first catalytic region.

In still other embodiments the sequestration agent is calcium carbonateand the catalyst is a carbonic anhydrase or a carbonic anhydrase analog.

In yet other embodiments the sequestration agent is a non-carbonatesequestration agent.

In still yet other embodiments the aqueous solution is one of either abrine solution or freshwater.

In still yet other embodiments the apparatus further includes at leastone enzyme separation filter configured to filter at least catalystpassing therethrough, the separation filter being in fluid communicationwith at least one output of the at least one reactor vessel.

In still yet other embodiments the apparatus further includes at leastone particle filtration system configured to filter at leastsequestration agent passing therethrough, the particle filtration systembeing in fluid communication with at least one output of the at leastone reactor vessel. In some such embodiments the particle filtrationsystem comprises a settling chamber in fluid communication with the atleast one reactor vessel.

In still yet other embodiments the sequestration agent is formed intograins of 100 micrometers or less.

In still yet other embodiments the apparatus further includes first andsecond reactor vessels, and wherein an input of the second reactorvessel is in fluid communication with the at least one output of thefirst reactor vessel, the second reactor vessel being arranged such thatan effluent from the first reactor vessel is delivered within theenclosed volume of the second reactor vessel to a second vesselcatalytic region wherein a second catalyst is disposed encompassing asecond vessel interfacial catalytic region located within a laminar flowboundary at the interface between the effluent and a second carbonatesequestration agent. In some such embodiments the temperature, pressureand pH of the two reactor vessels are independently variable. In someother such embodiments the second reactor vessel has a lower temperaturethan the first reactor vessel.

In still yet other embodiments the apparatus further includes a mixingchamber in fluid communication with the inlet of the at least onereaction vessel and wherein the CO₂ gas and aqueous solution inputs aremixed prior to introduction into the at least one reaction vessel.

In still yet other embodiments an effluent comprising at least unreactedCO₂ from the at least one output is reintroduced into one of the atleast one inputs of the reaction vessel.

In still yet other embodiments an effluent from the at least one outputhas a partial pressure of CO₂ lower than the partial pressure of CO₂introduced into the at least one input.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention, wherein:

FIG. 1 provides a schematic representation of the rate of dissolution ofcalcite against pH.

FIG. 2 provides a schematic flow chart of a carbon sequestration processin accordance with embodiments of the invention.

FIG. 3 provides a data graph plotting the rate of sequestration versuscalcite undersaturation in accordance with conventional sequestrationmethodologies and embodiments in accordance with the invention.

FIG. 4A provides a data graph plotting pCO₂ versus dissolved inorganiccarbon (DIC) for different values of Alkalinity (Alk); and

FIG. 4B provides a data graph plotting mineral undersaturation versusdissolved inorganic carbon (DIC).

FIG. 5 provides a data graph adapted from Cubillas (Cubillas et al.,Chem. Geo., 216:59 (2005)) plotting dissolution rate of calcite versuspH.

FIG. 6 provides a data graph plotting the solubility of carbonatesversus Mole % of Mg.

FIG. 7 provides a data graph plotting the calcite dissolution ratesversus mineral undersaturation (1−Ω).

FIG. 8 provides a data graph adapted from Barnes (Barnes (ed.), Barnes(ed.), Hydrothermal Ore Deposits, (1997), the disclosure of which isincorporated herein by reference) plotting calcite solubility as afunction of temperature and pressure of CO₂.

FIG. 9 provides a data graph plotting the dissolution rate of calcite asa function of saturation state and phosphate concentration.

FIG. 10A provides a data graph plotting the effects of pressure oncalcite saturation state.

FIG. 10B defines the effect of pressure on dissolution rate for a givendegree of undersaturation.

FIG. 11 provides a data graph adapted from Coto et al. (Coto et al.Fluid Phase Equilibria 324:1-7 (2012), the disclosure of which isincorporated herein by reference) plotting calcite solubility as afunction of salinity.

FIG. 12 provides a schematic flow chart of a carbon sequestrationprocess in accordance with a conventional system.

FIG. 13A provides a schematic of a one vessel carbon sequestrationapparatus in accordance with embodiments of the invention.

FIG. 13B provides a schematic of a two vessel carbon sequestrationapparatus in accordance with embodiments of the invention.

FIG. 14 provides a schematic diagram of an enzyme separation filter foruse in a process for carbon sequestration in accordance with embodimentsof the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, processes and apparatus for carbonsequestration utilizing a controlled catalyzed sequestration agentreaction in accordance with embodiments of the invention are provided.In many embodiments the system and method for carbon sequestrationincludes the controlled catalysis of the dissolution of a sequestrationagent, such as a mineral sequestration agent (e.g., calcium carbonate),to provide facile and permanent sequestration of CO₂ from the gas phase.The process and apparatus comprising the controlled catalysis inaccordance with many embodiments allows for the production of aneffluent outflow wherein the concentration of CO₂ is lower than thatfound in the reactant inflow. In embodiments, the controlled catalysisprocesses and apparatus produce a sequestration agent dissolution rateat least greater than the sequestration agent dissolution rate of anuncatalyzed system at the same mineral undersaturation. In variousembodiments, the controlled catalysis processes and apparatus produce asequestration agent dissolution rate at least one order of magnitudegreater than the sequestration agent dissolution rate of an uncatalyzedsystem at the same mineral undersaturation. In other embodiments, thesequestration agent dissolution rate is at least two orders of magnitudegreater than the sequestration agent dissolution rate of an uncatalyzedsystem at the same solution mineral undersaturation. In still otherembodiments, the sequestration agent dissolution rate is at least threeorders of magnitude greater than the sequestration agent dissolutionrate of an uncatalyzed system at the same solution mineralundersaturation. In various embodiments the mineral undersaturation isheld at near-equilibrium. In some such embodiments the sequestrationagent dissolution involves a carbonate to carbonate ion dissolution. Inother such embodiments the dissolution may include one or moresequestration agents including, for example, calcite, aragonite,dolomite and vaterite, silicate mineral etc.

Methods for carbon storage may further involve at least reaction ofdissolved CO₂ with a sequestration agent (such as, for example, calciumcarbonate, aragonite, dolomite, vaterite, etc.) under controlledcatalysis conditions to increase the dissolution rate of thesequestration agent at a fixed mineral undersaturation. In embodiments,the concentration of the catalyst in such reactions can be tuned tomaximize local protons at the sequestration agent surface to speed thedissolution of the sequestration agent, such as, for example, via one orboth of the replenishment of 1) bound protons such as hydrated CO₂(i.e., carbonic acid); or 2) free protons (e.g., from the protolysis ofwater) from the aqueous CO₂ reservoir or H₂O reservoir in proximity tothe sequestration agent. In many such embodiments, the methods includethe controlled catalytic formation of protons to speed the sequestrationagent dissolution, and thus the sequestration reaction of sequestrationagent with dissolved CO₂.

Many embodiments of methods and apparatus comprise the formation oftwo-controlled catalysis regions. In many such embodiments, catalyticmaterials (e.g., enzymes such as carbonic anhydrase) may be introducedin a controlled manner as catalysts to a reactor vessel containing asequestration agent and a feedstock of aqueous CO₂ to create a firstcatalysis zone or region that encompasses a second interfacial catalysisregion proximal to the sequestration agent (e.g., at the interface ofthe sequestration agent and/or within a laminar boundary layer about thesequestration agent) such that the barrier between the catalysis regionand the interfacial catalysis region is minimized.

The kinetics of sequestration reactions in embodiments comprising suchtwo-controlled catalysis regions may be configured in accordance with anumber of parameters, which may be independently controlled. In someembodiments of such engineered catalysis methods, the concentration ofthe catalyst in either or both the catalysis region or the interfacialcatalysis region can be configured to optimize the local protonconcentration at the interfacial catalysis region, such as, for example,by one or both the replenishment of H₂CO₃ (dissolved CO₂) from the CO₂aqueous reservoir and/or maximizing the protolysis of water. In otherembodiments the temperature and/or pressure within the catalysis regionand/or interfacial catalysis region may be increased to increasedissolution rate of the sequestration agent, at a fixed mineralundersaturation, as described above. In some embodiments theconcentration gradient of the various reactants (e.g., protons and CO₂)can be configured such that a maximum amount of reactant is deliveredwithin the interfacial catalysis region such that in some embodimentsthe reaction is diffusion limited by the delivery of reactants withinthe interfacial catalysis region rather than their removal from theinterfacial catalysis region.

Turning to the sequestration agent, in some embodiments thesequestration agent may be a mineral material such as for example,carbonates (e.g., metal carbonates), silicates, etc. Suitable calciumcarbonates may include any number of such mineral carbonate species,such as, for example, calcite, aragonite, dolomite, vaterite, etc. Invarious embodiments, one or more such sequestering agents may be used incombination. Accordingly, many embodiments use metal carbonatesequestration agents, while some embodiments use non-carbonatesequestration agents such as for example silicate minerals, and in somefurther embodiments, sequestration agents include various mineraladmixtures. Finally, in yet other embodiments, sequestration agents area combination of carbonate and non-carbonate solids. Regardless of thespecific sequestration agent(s) used, the particle size of sequestrationagent may be configured to optimize the surface area of the material,such that, for example, the surface area is increased to increase theexposure of the interfacial catalysis region to the catalysis region,and thereby increase dissolution of the mineral, and minimize theboundary layer between the catalysis region and the interfacial region.Exemplary embodiments utilized milli- or micro-meter scale particles ofsuch sequestration agent materials. Finally, in other embodiments themineral undersaturation of the sequestration agent can be tuned viamanipulation of the temperature, pressure, salinity and/or CO₂ (aq)concentration within one or both the catalysis region and interfacialcatalysis region, as will be discussed in greater detail below.

In many embodiments, an apparatus may be used to implement a carbonsequestration process as described above. In various embodiments, theapparatus may be comprised of at least a control system to monitor andadjust relevant parameters and at least one reactor whose inputs mayinclude at least CO₂ dissolved in an aqueous solution, carbonate, and acatalyst, where the reactor is adapted to form a two-region controlledcatalysis zone, wherein a first catalysis region containing a controlledconcentration of catalyst encompasses a second interfacial catalysisregion in proximity to a sequestration agent (e.g., the laminar boundarylayer surrounding the sequestration agent) such that the kinetic rate ofhydration and sequestration of carbon are controllable. In many suchembodiments, the reactor and the catalysis and interfacial catalysisregions are adapted to catalyze the production of protons (e.g., boundprotons or free protons) in the proximity of a sequestration agent(e.g., a calcium carbonate, metal carbonate, silicate, mineral clay,etc.) to increase the rate of dissolution of the sequestration agent andthe concomitant production of sequestration agent ions (such ascarbonate ions from a metal carbonate such as calcium carbonate. Inseveral embodiments the apparatus is configured to catalyze thedissolution of calcium carbonate in the presence of protons (i.e., freeor bound such as carbonic acid) to form calcium and bicarbonate. In manyembodiments the catalysis may operate to increase the protolysis ofwater in the reaction vessel and/or the formation of dissolved CO₂(i.e., carbonic acid).

The reactants, and configuration of such apparatus may be furtheroptimized for each specific sequestration catalysis. Accordingly:

-   -   In some embodiments, the aqueous solution may include at least        freshwater or seawater and may have a circum-neutral pH.    -   In further embodiments, the catalyst may include any material        suitable for the production of protons from the aqueous CO₂        solution, including, for example nickel materials and enzymes        such as, carbonic anhydrase or a modified carbonic anhydrase,        such as may be known in the art.    -   Within the reactor, there may be a mixing zone where the        diffusion boundary layer surrounding the sequestration agent        (e.g., carbonate) is decreased (e.g. by stirring, mixing the        aqueous solution and/or placing it in a fluidized bed).    -   Embodiments may contain particles of the sequestration agent in        varying sizes. In some exemplary embodiments micrometer scale        sequestration agent particles may be used; while in other        embodiments larger sequestration agent slabs or beds may be        formed.    -   In some embodiments, metallic carbonates, mineral clays or        silicates may be used as a sequestration agent.    -   Some embodiments may include a feedback loop that controls and        continually adjusts mineral undersaturation and the input of        catalyst, CO₂, and a sequestration agent (such as, for example,        CaCO₃) such that two controlled and active catalytic regions are        preserved within the reaction vessel in the vicinity of the        sequestration agent (e.g., at the interface or within a laminar        boundary layer about the interface thereof and surrounding such        interfacial region) and the sequestration rate efficiency within        such catalytic regions are maximized.    -   In the reactor and the optional feedback loop, at least        parameters such as pH, catalyst concentration, and CO₂        concentration may also be monitored in embodiments of the        invention.

In many embodiments of these systems, one or more of such processparameters may be adjusted such that a rate of carbon sequestrationlimited only by the diffusion rate of the carbonate ions is achieved,i.e., that the reaction is limited by product transport. In otherembodiments the parameters may be controlled to provide an excess ofprotons: either bound, such as in the form of hydrated CO₂ (e.g.,carbonic acid) or free, such as from the protolysis of water, such thatthe sequestration reaction rate is limited by the diffusion of protonsand CO₂ to the sequestration agent surface, i.e., that the reaction islimited by reactant transport.

Additionally, some embodiments of the invention may also include asecond reactor parameterized separately to further neutralize or degasany residual CO₂. In embodiments using this second reactor, the inputmay be the aqueous solution discharged from the first reactor. Thesecond reactor parameters including at least CO₂ concentration, aqueoussolution input, carbonate saturation, and catalyst input may bemonitored and separately controlled in some embodiments. In still otherembodiments the apparatus may contain any number or configuration ofreaction vessels any one or more of which may be provided with thetwo-catalysis region configuration in accordance with embodiments.

In further embodiments, filters and other devices are used to retainsequestration agent and/or catalyst within a reactor vessel. Forsequestration agent retention, in some embodiments, sequestration agentis retained by filtering effluent water using a particle filtrationsystem. In yet other embodiments, sequestration agent is retained usinga settling chamber, which allows reactant to settle so that solid-freeeffluent water may be discharged. In such embodiments, the settlingchamber may be part of a reactor vessel or may be a separately dedicatedvessel. In yet further embodiments, a combination of sequestration agentretention strategies may be used. In many embodiments, catalyst isretained using various filtration systems. In some embodiments, catalystis attached to free-floating solid beads which are retained in thereactor using a particle filtration system or a settling chamber, asdescribed above for the sequestering agent. In some other embodiments,dissolved catalyst is retained using enzyme separation techniques. Instill other embodiments, catalyst is replenished usingcatalyst-expressing organisms, which may be located within or outsidethe reaction vessel. Such embodiments may also include methods andapparatus for removing/filtering harmful metal or other contaminantsthat might be contained within the effluent.

In various embodiments, a feedstock enriched in CO₂ reacted inaccordance with embodiments produces an outflow that has a reducedpartial pressure of CO₂. In other embodiments the reactor effluent mayhave slightly enhanced alkalinity, which may be environmentallyfavorable. In embodiments using seawater, this effluent may bedischarged into coastal environments, which may mitigate harmful‘acidified’ waters. In embodiments using freshwater and/or brines (e.g.seawater), the effluent may optionally be used for agricultural orcommercial uses. In other embodiments the catalyst may be bound to asolid surface, such that the catalyst is not discharged into theenvironment along with the products from the reaction, or alternativelythe catalyst may be filtered out of the effluent. In some embodimentssome or all of the active catalyst is allowed to remain in the effluent.In other embodiments catalyst remains in the effluent but is deactivatedby heating or other means. In some embodiments the effluent isdischarged into the environment. In still other embodiments effluent canbe re-enriched with CO₂ and return to the reaction vessel one ormultiple times before it is finally discharged.

Finally, in accordance with still other embodiments of the invention,rates of CO₂ hydration for aqueous (or dissolved) CO₂ can be alteredthrough one or more methods including but not limited to introduction ofa catalyst, altering the flow of the inlet gas stream (i.e., CO₂) andincreasing the surface area of aqueous solution in contact with a givenvolume of gaseous CO₂.

Review of Carbon Sequestration Approaches

Carbon sequestration involves two steps: carbon capture (impermanentremoval of carbon from the atmosphere) and carbon storage (permanentremoval from the atmosphere). To mitigate rising levels of CO₂, thescale of carbon sequestration must be commensurate with emissionslevels. Currently, anthropogenic carbon emissions are roughly 40gigatons (Gt) of carbon dioxide per year. Because of the magnitude ofemissions, for a sequestration strategy to be viable it must be able tokeep up with rates of anthropogenic emissions. The carbon sequestrationstrategies currently available do not permanently sequester carbon atrates sufficient to match amounts of anthropogenic carbon dioxideemissions.

Many carbon sequestration strategies are inadequate because while theyprovide methods of temporarily capturing CO₂, they do not providemethods to permanently store CO₂ to prevent it from being released backinto the atmosphere. Because carbon sequestration requires both carboncapture and storage, these strategies cannot be said to truly sequestercarbon. Examples of carbon sequestration strategies that capture andonly temporarily store CO₂ are those that primarily involve thedissolution of gaseous CO₂ and the hydration of aqueous CO₂. DissolvingCO₂ gas into water is ineffective for sequestration because once thestorage water contacts the atmosphere, the CO₂ in solution will begin todegas out of solution. Moreover, this process is relatively rapid,occurring in some cases over a period of months to weeks to hours. As aresult, and as will be discussed in greater detail below this rapiddegassing requires secondary storage requirements, such as, for example,ground water injection to prevent release of carbon back into theenvironment.

The reason for the temporary nature of many conventional methods ofcapturing CO₂ relates to the nature of how they are attempting to“store” CO₂. In particular, the general strategy in these methods is toincrease the “hydration” of CO₂ (i.e., the dissolution of CO₂ intosolution thereby increasing the amount of CO₂ that, in theory, is beingtaken out of the atmosphere. In true hydration, carbon is captured ascarbonic acid (H₂CO₃), in accordance with the reaction below,CO ₂ +H ₂ O

H ₂ CO ₃.However, the reaction dynamics of dissolving CO₂ into solution arecomplicated and the true hydration of CO₂ to carbonic acid (H₂CO₃) is arelatively minor component of the overall reaction. Specifically, thehydration of CO₂ (i.e., the formation of carbonic acid) is very slow,and this hydration is also much slower than the dehydration of CO₂,i.e., the reversion of carbonic acid to CO_(2(aq)). This is because thereaction thermodynamics are highly unfavorable to the formation ofH₂CO₃. Accordingly, a solution of hydrated CO₂ usually contains mostlyCO_(2(aq)), with very small amounts of H₂CO₃. For example, the value forK_(eg) for H₂CO₃ and CO_(2(aq)) is 0.0015 at 25° C. and 1 atmospherepressure. Accordingly, the ratio of CO_(2(aq)):H₂CO₃ at room temperatureis around 670. For this reason, sequestration strategies that focus on“hydration” largely result in solutions of CO_(2(aq)). Accordingly, theterm H₂CO₃* is often used to describe the aqueous mixture of CO₂ andH₂CO₃ formed by such hydration reactions, where H₂CO₃* is the sum of theother two species. In short, many sequestration strategies that focus onhydration involve the formation of H₂CO₃*, not H₂CO₃.

Furthermore, because the thermodynamics are so unfavorable, if asolution contains large amounts of CO_(2(aq)) for the purpose of drivinghydration, degassing is likely to occur. As a result, there is highlikelihood that CO_(2(aq)) will leave solution and return to gaseous CO₂when it contacts the atmosphere. Moreover, as [CO_(2(aq))] decreases dueto degassing, kinetics also becomes a factor making hydration even lessfavorable. Thus the dissolution of CO₂ into solution, or the “hydration”of CO₂ by itself does not constitute sequestration because it is adynamic process where CO₂ is constantly dissolved into and released backinto the environment, and so does not effectively store the reacted CO₂in a permanent manner, unless the fluid is permanently isolated from theenvironment, such as by being injected into an impermeable storagereservoir. The drawback of such systems is that they are subject toleaks and so require constant monitoring and mitigation measure todetect and prevent leakage back into the environment.

To more effectively store CO₂ permanently it can be reacted withsomething to chemically fix the CO₂. Examples of sequestering CO₂ inthis manner include organic carbon fixation (i.e., photosynthesis) andreaction with a sequestration agent such as limestone. Carbon fixationfrom photosynthesis is the fixation of gaseous CO₂ in a series ofreactions known as the Calvin cycle. However, even this photosyntheticreaction does not provide a permanent way to fix carbon because thetrees or plants into which the CO₂ is being stored will eventually dieand when they do, the carbon stored in the plant matter will return tothe atmosphere through respiration by termites and microbes. Incontrast, through reaction with limestone (CaCO₃), the planet willnaturally consume nearly all of the anthropogenic CO₂ emissions in thefollowing reaction:CO ₂ +H ₂ O+CaCO₃

Ca ²⁺+2HCO ₃ ⁻In nature, this reaction takes place in the deep ocean whereundersaturated waters are in contact with carbonate bearing sediments.However, the kinetics of this reaction are very slow. There are kineticlimitations in the ocean-atmosphere-sediment system, some of which arerelated to reactant transport (Archer et al., Global Biogeochem. Cy12:259-276 (1998), the disclosure of which is incorporated herein byreference). Initially, the exchange of CO_(2(g)) with CO_(2(aq)) at theocean surface will take a few hundred years. Next, the CO_(2(aq)) at thesurface must travel from the surface to the deep ocean, a process thattakes an order of 1000 years. And finally, when the CO_(2(aq)) reachesocean sediments, it must react with CaCO₃ which occurs on a timescale ofseveral thousand years. In total, it is estimated that the e-foldingtimescale for these processes is 6800 years (Archer et al., 1998, citedabove). As can be seen, both of these natural reaction processes haveserious drawbacks related to reactant transport and will not beeffective on the types of anthropogenic time-scales necessary to addressanthropogenic CO₂ emissions.

Apart from the kinetic challenges associated with reactant transport inthe natural environment, dissolution itself is very slow. The followingequation illustrates the hydration of CO₂ and how concentrations ofcarbonic acid and carbonate ions are closely linked by simple acid-basereactions:CO ₂ +H ₂ O

H ₂ CO ₃

⁺ +HCO ₃ ⁻

2H ⁺ +CO ₃ ²⁻The kinetics of calcium carbonate dissolution are typically described bythe equation:Rate=k(1−Ω)^(n)where Ω is a mineral's saturation state, which is defined as the productof effective in situ calcium and carbonate ion concentrations divided bythe apparent solubility product for that mineral ([Ca²⁺][CO₃²⁻]/K′_(sp)); k is a rate constant; and n is the reaction order. Thereaction order has no physical or chemical significance; it solelydescribes an empirical relationship between saturation state and thedissolution rate. In this set of equations thermodynamic potential, ormineral undersaturation, (1−Ω) drives the dissolution rate.

Attempts have been made to augment the kinetics of carbonatedissolution, but thus far have not succeeded in designing effective orpracticable solutions. For example, attempts were made to increase thegeological rate of dissolution observed in the water column by reactingCO₂ with limestone at the surface of the earth thereby removing thedelay caused by the transport of reactants from the surface of the oceanto the bottom of the ocean. In a process described in U.S. Pat. No.7,655,193, a reactor charged with limestone pellets and designed to keepa steady stream of CO₂ dissolved in seawater might reasonably be able tomaintain 30% mineral undersaturation (equivalent to equilibratingseawater with gas containing about pCO₂ of about 4,000 ppm) to achieve adissolution rate of 6·10 g/cm²/day (Subhas et al. Geochimica etCosmochimica Acta, Volume 170, 1. (2015), pgs 51-68, the disclosure ofwhich is incorporated herein by reference). However, even using veryhigh [CO₂] gas (10% v/v) and a 1-2 week reaction time, the processdisclosed in U.S. Pat. No. 7,655,193 only manages to achieve adissolution rate of 6·10⁻⁵ g/cm²/day. (See, e.g., Rau, G. H. (2011).Environmental Science & Technology, 45(3), 1088-1092, the disclosure ofwhich is incorporated herein by reference.)

Operating on a global scale, the aforementioned rates of carbonatedissolution are simply too slow to adequately address the current levelsof CO₂ emissions. Applied to the sequestration of 40 Gt of CO₂ emittedannually, such a system would require a large amount of calciumcarbonate to be dissolved in tens to hundreds of thousands of reactorfactories around the world. For example, dissolving enough limestone toannually titrate 40 Gt of CO₂ using a conventional system would requirea cube of limestone that is roughly 2 miles on each side. Daily, thiswould require 2.5·10¹⁴ g of CaCO₃ to be dissolved. Using reactorssimilar to those described in U.S. Pat. No. 7,655,193 having a 2 m³volume, containing 1 m³ of CaCO₃ broken up into 1 mm³ cubes, and at 30%mineral undersaturation, the dissolution reaction would require 70billion reactors. To further illustrate the scale of implementing asequestration method operating at this rate, if the 2 m³ reactors wereplaced in factories sized 300 m×600 m×10 m, this would still require767,000 factories. Operating at the maximum rate describe in the U.S.Pat. No. 7,655,193 patent, the number of factories would only decreaseby a factor of 10. As can be seen from the above illustration, the sheermagnitude of the amount of CO₂ to be dissolved requires a faster rate ofdissolution for this method of sequestration to be practical.

Mineral undersaturation can be achieved by increasing [CO_(2(aq))],decreasing solution temperature, increasing solution pressure, and/orpumping pressurized CO₂ gas into the aqueous solution to allow forgreater dissolution of CO₂. However, there are other impracticalitiesrelated to relying on high concentrations of CO_(2(aq)) alone toincrease reaction rates. In particular, the type of cool, pressurized,high [CO_(2(g))] gas streams required need significant inputs of energyto create. To save energy, sequestration could be integrated intopreexisting industrial processes. In an industrial setting, however, oneis unlikely to encounter cool, pressurized, high [CO_(2(g))] gaseffluent streams, because most effluent streams that are rich in CO₂ areunlikely to be cool. On the other hand, with high temperature streams,it is difficult to keep the CO₂ in solution without expending largeamounts of energy, which may negate any of the benefits of thesequestration strategy related to carbon emissions. Therefore, relyingon gaseous undersaturation alone to drive sequestration would lead to avery inefficient and impractical process.

Embodiments of the present invention are directed to systems, methodsand processes that use a carefully engineered catalysis reaction at theinterfacial reaction zone between hydrated CO₂ and a sequestration agentto increase the rate of sequestration agent dissolution for fastersequestration, providing a feasible approach to sequestration agentdissolution. In some embodiments of the proposed invention, thesequestration rate may be made independent of the saturation state ofthe mineral such that far from equilibrium mineral undersaturation isnot necessary to increase reaction rates substantially above thosepresently described in the art. As a result, embodiments of theinvention can be more practically implemented than others requiring highmineral undersaturation.

Description of Catalyzed Sequestration Parameters

The mechanism and rate of sequestration is typically defined by the rateof carbonate production. As schematized in FIG. 1, the rate of carbonatedissolution in seawater depends directly on the alkalinity/pH/DIC of thesolution (e.g., mineral undersaturation). At low alkalinity (low pH,e.g., Region 1 and below), the rate of carbonate production is limitedto proton attack only. By contrast, as the Alkalinity:DIC ratioincreases (where mineral undersaturation is not so high and CO₂ is inequilibrium with alkaline water, e.g., Region 2) the uncatalyzeddissolution rate begins to decline independent of transport control. At“circum-neutral” pH ranges (e.g., between regions 2 and 3) there is anextreme drop-off, which generally prevents the efficient sequestrationof CO₂ within these high pH (e.g., near-equilibrium undersaturation)regimes. In embodiments of this invention it has been found thatprotons, and more specifically the presence of either free (e.g., fromprotolysis of water) or bound (e.g., carbonic acid) protons introducedinto a catalysis reaction region in proximity to a sequestration agent,increases the dissolution rate of the sequestration agent and plays animportant mechanistic role in the permanent sequestration of CO₂,particularly in environments like seawater, such that sequestrationrates can be increased such that they are, in some embodiments,transport limited even in equilibrium undersaturation regimes (e.g.,circum-neutral pH ranges). In other words, it has now been discoveredthat dissolution rates in Region 3 can be enhanced by utilizing thecontrolled catalysis methods and apparatus in accordance withembodiments to behave more like dissolution rates in Regions 1 and 2.

Conventional mechanistic models have focused on specific carbonatespecies and their interaction with the mineral surface. In general,these mechanistic equations are made up of multiple terms, each withfirst-order dependence. (See, e.g., Plummer, L. N., Wigley, T., 1976.Geochimica et Cosmochimica Acta 40 (2), 191-202; Shiraki, R., Rock, P.A., Casey, W. H., 2000. Aquatic Geochemistry 6, 87-108; Gledhill, DwightK., and John W. Morse. Geochimica et cosmochimica acta 70.23 (2006):5802-5813; Finneran, David W., and John W. Morse. Chemical geology 268.1(2009): 137-146; Arakaki, Takeshi, and Alfonso Mucci. Aquaticgeochemistry 1.1 (1995): 105-130, the disclosures of which areincorporated herein by reference.) The dissolution of calcite in dilutesolution can be described using three main regimes: onetransport-controlled regime dominated by hydrogen ion attack anddiffusion, and two surface-controlled regimes controlled by proton(either H₂CO₃ (carbonic acid) or water) attack. As described above withrespect to FIG. 1, different sequestration systems may harness differentreaction mechanisms. In solutions of high pH and low pCO₂ (likeseawater, for example), the Plummer and Wigley's 1976 study proposedthat water attack is the main dissolution mechanism. It was also thoughtthat H₂CO₃ attack was valid at high pCO₂. Later, it was thought that amulticomponent dissolution mechanism in which hydrogen ion, bicarbonateion, and hydroxide ion all function as dissolving agents in different pHregimes was the best model (Shiraki, 2000). In seawater conditions thedominant dissolution mechanism was thought to be bicarbonate attack,where a bicarbonate ion acts as a nucleophile, and performs anucleophilic attack on a hydrated calcium ion on the mineral surface.Despite the fact that H₂CO₃ is the most acidic of the carbonate species,it was not implicated as part of the standard seawater dissolutionmechanism. Another mechanistic model proposed that in the region ofseawater pH, H₂O and H₂CO₃ activities determine the dissolution rate ofcalcite (Plummer and Wigley, 1976). Here again, the role of CO₂hydration kinetics in dissolution rate was not considered.

In particular, in these conventional methods, the kinetics of CO₂hydration and its relationship to solid phase dissolution rate were notconsidered. By contrast, in embodiments of sequestration systems,methods and apparatus, proton species (free or bound such as H₂CO₃) havenow been identified as the dominant species involved in the dissolutionof sequestration agent, and methods and apparatus are provided topersistently increase its standing stock in proximity thereto. In short,true rates of proton production (e.g., hydration of CO₂ to carbonic acidor protolysis of water) are catalytically enhanced, and then the protonconcentration is preserved and controllably presented directly at thesource of sequestration agent to maximize the dissolution rate at afixed mineral undersaturation, and particularly at mineralundersaturations previous thought to be so close to equilibrium so as tobe too slow. Thus, in embodiments where the sequestration agent is acarbonate, such as, for example, CaCO₃, mechanistically the dissolutionof the carbonate sequestration agent can be expressed as:CaCO_(3(s)) +H ₂ CO ₃ →Ca ²⁺+2HCO ₃ ⁻Depending on in situ pH and other factors, the reaction products mayinclude carbonate ion (HCO₃ ⁻) or alternatively metal carbonate (suchas, for example, calcium bicarbonate (Ca(HCO₃)₂)). For the purposes ofthis application, carbonate ion and bicarbonate may be usedinterchangeably.

The chemical underpinning of this reaction is fundamentally differentthan previous attempts at carbon dioxide sequestration both conceptuallyand in application, Using engineered catalysis schemes, embodiments ofthe system, process and apparatus increase the rate of conversion ofCO2(aq) and water to active dissolution agents (i.e., protons) at thesource of sequestration agent without necessarily requiring aconcomitant increase in the mineral undersaturation allowing for adramatic increase in the overall rate of CO₂ sequestration achievable.

Though the presence of protons (free or bound) has now surprisingly beendiscovered to be the dominant species involved in carbonate dissolution,they are often in short supply because, as described above, hydration isa very slow kinetic step. The hydration reaction can be expressed in thefollowing equation:CO _(2(aq)) +H ₂ O≈H ₂ CO ₃

Uncatalyzed, in an aqueous solution, the rate of dehydration has beenshown to exceed the rate of hydration by a factor of ˜1000 under someconditions (Wang, Xiaoguang, et al. The Journal of Physical Chemistry A114.4 (2009): 1734-1740, the disclosure of which is incorporated hereinby reference). Thermodynamically, then, there is a very strong tendencyfor H₂CO₃ to dissociate to CO_(2(aq)) and, if exposed to the atmosphere,for CO_(2(aq)) to degas to CO_(2(g)). Carbonic acid can also undergoacid-base reactions to form bicarbonate (HCO₃ ⁻) and carbonate (CO₃ ²⁻)ions. So, for a fixed concentration of H₂CO₃*, the equilibrium abundanceof CO_(2(aq)) outweighs the abundance of H₂CO₃ by a very large factor.

Accordingly, systems that focus on increasing H₂CO₃* for sequestrationwill be unable to sequester carbon at higher rates, and because ofthermodynamics, the standing stock of H₂CO₃ will likely to be very low.The standing stock will be even lower if dissolution of a sequestrationagent (such as carbonate) consumes H₂CO₃ much faster than it isproduced. A relatively low standing stock of H₂CO₃ will, in thisexample, prevent higher rates of carbonate dissolution and thus lowerthe kinetic rate of sequestration.

Certain enzymes have been shown to catalyze the hydration of CO₂. Theseenzymes include, but are not limited to, carbonic anhydrase. The actualmechanism of carbonic anhydrase was shown by Silverman and Lindskog(1988) to essentially catalyze the protolysis of water:H ₂ O+CO _(2(aq)) ↔OH ⁻ +CO _(2(aq))+enzyme-H ⁺ ↔HCO ₃ ⁻+enzyme-H ⁺ ↔H ₂CO ₃+enzyme,where the “enzyme” is carbonic anhydrase, but could be, any molecule ormaterial that performs this set of chemical transformations. The enzymetransports the proton away from the active site to the solution, whereit can later react with the newly formed bicarbonate ion. Thus theenzyme does at least four critical things: (1) it protolyzes water intohydroxide ion and protons; (2) it transports the proton away from theactive site to the bulk solution; (3) it reacts carbon dioxide withhydroxide ion to form bicarbonate; and (4) it releases bicarbonate tolater recombine to form carbonic acid. This gives the reaction mechanismwith carbonate two options. In one, carbonate (such as for examplecalcium carbonate) dissolves through interaction with an enzyme (such asfor example carbonic anhydrase), for example:CaCO_(3(s)) +H ₂ CO ₃ →Ca ²⁺+2HCO ₃ ⁻In another, carbonate dissolves through interaction with protons atcircum-neutral pH, for example:CaCO_(3(s)) +H ⁺ +HCO ₃ ⁻ →Ca ²⁺+2HCO ₃ ⁻

In view of the above, embodiments of the proposed invention rely on anovel kinetic and mechanistic formulation of carbonate dissolution toprovide an engineered system with enhanced rates of carbonatedissolution. Embodiments, methods and apparatus, are providedincorporating catalysis schemes configured to provide highconcentrations of protons, (e.g., free protons or bound protons, such ashydrated CO₂) in proximity with a sequestration agent, such as, forexample, a carbonate, such that the kinetics of hydration and carbonatedissolution are enhanced, while the mineral undersaturation (1−Ω)remains fixed (i.e., does not require high levels of undersaturation orundersaturation far from equilibrium). In some embodiments,concentrations of carbonic acid and or free protons are fine-tuned in aninterfacial catalysis region proximate to the sequestration agent (e.g.,carbonate surface) interface (e.g., within the laminar boundary layer ofthe sequestration agent surface). This is to be contrasted withconventional CO₂ hydration systems, which rely on the assumption thatdissolution is driven solely by gaseous saturation state of aqueous CO₂,i.e., by trying to increase mineral undersaturation. Accordingly,embodiments of the methods and apparatus of the current system usingtwo-region the controlled catalytic methods are capable of driving thereaction rate of CO₂ sequestration even in the absence of strong mineralundersaturation.

Catalyzed Sequestration Process Embodiments

Many embodiments of the invention are directed to sequestrationprocesses that provide increased rates of carbon sequestration bycreating at least two controlled catalyzed regions in proximity to asequestration agent (such as CaCO₃). A schematic of a process inaccordance with embodiments utilizing a CaCO₃ sequestration agent isprovided in FIG. 2 and will be discussed in greater detail below. Inthese embodiments, for example, the slow reaction steps of hydration anddissolution are enhanced to provide an excess of hydrated CO₂ in theform of carbonic acid at an interfacial region of the sequestrationagent to maximize mineral dissolution rates at a fixed mineralundersaturation. In embodiments, of this system, a reaction driven bythe presence of a suitable catalyst, such as, for example, carbonicanhydrase, controllably introduced into a reaction vessel, is utilizedto enhance the rate of proton production (e.g., water protolysis and CO₂hydration) and increase the bulk concentration of protons in solution ina specific catalyzed region such that the concentration of protonspresent within an second interfacial catalysis region (defined in someembodiments as a region surrounding the solid sequestration agent and inother as the laminar boundary layer of the sequestration agent) issufficient to maximize the dissolution of the sequestration agentindependent of the mineral undersaturation (e.g., at equilibrium mineralundersaturations).

To understand the operating principal behind embodiments of thesemethods and apparatus it is necessary to understand the differencebetween the conventional uncatalyzed approaches to CO₂ sequestration andthose disclosed in accordance with embodiments. FIG. 3, provides agraphical demonstration of these differences. As shown, in seawater, therate of carbonate dissolution can be enhanced by several orders ofmagnitude by conducting dissolution in the presence of a suitablecatalyst (such as, for example carbonic anhydrase), that is by creatinga controlled catalyzed region in proximity to the interface between thecarbonate sequestration agent and the CO₂ solution. Other systems usecatalysts like carbonic anhydrase to simply increase the concentrationof dissolved carbon dioxide, but in these systems there is potential fordegassing because sequestration does not occur immediately afterhydration. Embodiments of the claimed process use catalysis in afundamentally different manner. Instead of using catalysis to increase[H₂CO₃*], catalysis is used to produce protons (both free and in theform of hydrated CO₂) within an interfacial catalysis region surroundingthe sequestration agent thereby increasing the rate of mineraldissolution of the sequestration agent without the need for an increasein mineral undersaturation (e.g., a mineral undersaturation far fromequilibrium).

Examining the data in FIG. 3 in more detail it is shown that uncatalyzeddissolution can be extrapolated to complete mineral undersaturation toget a maximum possible rate of 2.5×10⁻³ g/cm²/day. The carbonate iontransport limitation is ˜3×10⁻³ g/cm²/day (assuming a boundary layerthickness of 10 microns). Accordingly, this data demonstrates that usingconventional sequestration schemes without the carbonate catalysissystems and methods in accordance with embodiments there must becomplete mineral undersaturation (which as will be discussed in greaterdetail below is an impossibility) to sequester CO₂ efficiently.

In contrast, FIG. 3 shows that dissolution rates for carbonate catalyzedsystems and methods in accordance with embodiments approach 1×10⁻⁴g/cm²/day to 1×10⁻³ g/cm²/day at mineral undersaturation (1−Ω) values offar from complete mineral undersaturation (e.g., from 0.5 to as low as0.1). Accordingly, using embodiments of the catalyzed systems andmethods described, catalysis rates can approach the physical maximumpossible in natural waters. As such, this data demonstrates that usingthe catalysis systems and methods and minimizing reactant transport, byoperating such catalysis within a defined catalysis region such thatcatalyzed reactant can interact within an interfacial catalysis regionin proximity with the sequestration agent, the systems and methodssequester carbon much faster than previously observed. Indeed, thereaction rate approaches the diffusion limit in a way that othersequestration methods, particularly those that emphasize CO₂ dissolutionand hydration, are simply not capable of attaining. Not to be bound bytheory, but in some embodiments it is possible through the use ofcatalysis in the system described to achieve sequestration rates thatreach the diffusion limit (e.g., in some embodiments exceed adissolution rate of 2.5 10⁻³ g/cm²/day). In many embodiments, thecatalysis can be run at an equilibrium mineral undersaturation (e.g., ata pH of 4.5 or higher and in some embodiments as high as 6 or higher)and achieve dissolution rates of from 1·10⁻⁴ g/cm²/day to 1·10³g/cm²/day.

As described above, in conventional sequestration systems it isnecessary to approach complete mineral undersaturation (i.e., a 1−Ωvalue of 1) to achieve the rates of sequestration obtained usingembodiments of the current catalyzed system. FIGS. 4A and 4B providecalculations that illustrate the difficulty in achieving completemineral undersaturation in seawater. At standard seawater ranges ofmineral undersaturation, dissolution kinetics are slow (1e-6 to 1e-5g/cm²/day). To achieve much higher rates of dissolution, 1−Ω mustapproach 1. The calculated data in FIGS. 4A and 4B show that this is notphysically possible. As observed in a theoretical calculation with CO₂injection at fixed alkalinity like those found in seawater (FIG. 4A), asDIC is increased the saturation state plateaus to a lower, but nonzero,value of Ω (FIG. 4B). Even at very large DIC enrichments (high pCO₂)(FIG. 4A), complete mineral undersaturation is impossible throughdissolving gaseous CO₂ into seawater alone. Accordingly, conventionalsystems that attempt to increase sequestration rates by increasing CO₂concentration in solution (i.e., gaseous undersaturation) simply are notcapable of reaching the rates of sequestration provided in the systems,methods and apparatus described in embodiments of the invention.

As described above with reference to FIG. 1, typically, the diffusionlimit is calculated for the transport of reaction products away from thesurface. In embodiments according to the systems, methods and apparatus,the diffusion limit may be calculated for transport of the sequestrationagent (e.g., metal bicarbonate, i.e., calcium bicarbonate) away from thesurface sequestration agent. If instead the reaction may be limited bythe transport of the reactants to the sequestration agent surface as insome embodiments of systems and methods in accordance with theinvention, it would be possible to speed up the sequestration of carboneven further. Although such a reactant transport limitation has beendescribed in other studies, these studies typically require an extremelylow pH for carbonate dissolution, through reaction with hydronium ion(as shown and described in FIG. 5, taken from Cubillas, P., Kohler, S.,Prieto, M., Chairat, C., Oelkers, E. H., 2005. Chemical Geology 216(1-2), 59-77, the disclosure of which is incorporated herein byreference. As further shown in FIG. 5, at low pH (below 5), thedissolution rate of carbonate is linearly related to H⁺ concentrationand is commonly interpreted as the transport limitation of H⁺ to themineral surface. FIG. 5 illustrates the difficulty in driving highdissolution rates at pH values greater than 5. Thus, this shows that,optimally, to increase dissolution (and hence sequestration) rates, thepH should be lowered as much as possible, preferably below 5.

In reality it is nearly impossible to drive seawater pH to values lessthan 5 with the addition of gaseous CO₂ alone. So, in other systemswhich emphasize maximizing [H₂CO₃*] in solution prior to reaction withcarbonate, rates of dissolution would be slow, consistent withdissolution rates for circum-neutral solution. Also, in media likeseawater it is difficult to drive mineral saturation values (Ω) to lessthan 0.2. Indeed, as shown and described in FIG. 4B, even with theaddition of acid (lower alkalinity), it is impossible to reach completemineral undersaturation. Accordingly, in conventional sequestrationsystems there must be complete mineral undersaturation to sequester CO₂efficiently. However, as discussed, using a controlled catalyzed systemas described in embodiments of the systems, methods and apparatus it ispossible to achieve high rates of carbon sequestration without requiringhigh levels of mineral undersaturation or extremely low pHs by insteadengineering a method and/or apparatus in which the sequestrationreaction occurs within a catalyzed region whereby protons (e.g., freeprotons or bound protons in the form of carbonic acid) are present at aninterfacial catalysis region surrounding the sequestration agent (e.g.,surface of mineral carbonate) to thus increase mineral dissolution,reduce the need for substantial reactant transport, and reduce thepossibility of dehydration of the carbonic acid and degassing of the CO₂from solution even at mineral undersaturation regimes near equilibrium.

Accordingly, although specific embodiments of methods and apparatus inaccordance with embodiments are described, regardless of the specificdesign of the reaction vessel and catalysis regions disposed within thereaction vessel, the controlled catalysis methods and apparatus form twocatalysis regions: a first catalysis region where catalyst and CO₂ (aq)can combine in a controlled manner and a second interfacial catalysisregion defined by the interface between the surface of the sequestrationagent and the surrounding solution (e.g., the laminar boundary layer ofat the sequestration agent surface). In such a two catalysis regionset-up the concentration of catalyst, CO₂ (aq), temperature andpressure, flow rate, CO₂ (gaseous) injection rate, etc. can be tunedto: 1) maximize the dissolution rate of the mineral, and 2) maximize thereplenishment of protons (e.g., free protons from the protolysis ofwater and/or H₂CO₃ from the CO₂(aq) reservoir) from the first to thesecond interfacial catalysis region. Using such a two-region catalysiszone reaction method and vessel, the concentration of protons in thesolution delivered to the sequestration agent can be maximized andreplenished at the sequestration agent (e.g., carbonate, mineral clay,silicate, etc.) surface. In many embodiments, such processes andapparatus produce a dissolution rate of sequestration agent within theinterfacial catalysis region greater than the dissolution rate ofsequestration agent in an uncatalyzed system at the same mineralundersaturation. In many embodiments, such processes and apparatusproduce a dissolution rate of sequestration agent within the interfacialcatalysis region at least one order of magnitude greater than thedissolution rate of sequestration agent in an uncatalyzed system at thesame mineral undersaturation. In other embodiments, the sequestrationagent dissolution rate within the interfacial catalysis region is atleast two orders of magnitude greater than the sequestration agentdissolution rate of an uncatalyzed system at the same mineralundersaturation. In still other embodiments, the sequestration agentdissolution rate within the interfacial catalysis region is at leastthree orders of magnitude greater than the sequestration agentdissolution rate of an uncatalyzed system at the same mineralundersaturation. In any of the above embodiments the mineralundersaturation of the system may be held near equilibrium. In stillother embodiments the system may be run in a circum-neutral conditions(e.g., at a pH greater than 4.5 and in some embodiments from pH 4.5 topH 7).

These principles are applied in an exemplary embodiment of a process(600), illustrated in FIG. 2 where the sequestration agent chosen isCaCO₃ (640). In FIG. 2, primary inputs into the system are seawater(610), a catalyst (620), and a sequestration agent (e.g., a metalliccarbonate, such as calcium carbonate, 640), although it will beunderstood that in other embodiments additional inputs (as describedabove) including, for example, additives to decrease contaminants,temperature, pressure, etc. may be provided, as will be described ingreater detail below. In the exemplary embodiment the seawater input 610is premixed to contain aqueous CO₂. It should be understood that manymechanisms to improve the mixing of water and CO₂ to form an aqueous CO₂are contemplated in embodiments. For example, the seawater may beaerosolized and/or agitation or other stirring mechanisms may beprovided. In addition, the pressure and temperature of the reactionvessel may also be controlled as necessary to increase reaction rates.Likewise, although specific sequestration agents and catalysts arediscussed in reference to FIG. 2, it will be understood that equivalentalternatives to these inputs may be chosen as will be discussed furtherbelow.

As previously discussed, due to the thermodynamics of hydration,seawater contains about one thousandth the amount of H₂CO₃ asCO_(2(aq)). In embodiments of the process, a catalyst (e.g., carbonicanhydrase) is combined with the seawater in a first catalysis region(657) which surrounds a second interfacial catalysis region (660) thatat least defines the interface between the solution in the firstcatalysis region (657) and the surface of a solid sequestration agent(in this example, carbonate) to form a controlled or engineeredinterfacial catalysis region (660). Within the first catalysis region(657), the combined seawater, catalyst (650) and sequestration agent actto increase replenishment of bulk protons (e.g., [H⁺] and [H₂CO₃]) (655)at the interfacial catalysis region sequestration agent above levelsfound in the absence of carbonic anhydrase, as discussed above inreference to the standard ratios of H₂CO₃ and CO_(2(aq)) in uncatalyzedH₂CO₃*. The first catalysis region is further engineered to deliver andreplenish the concentration of protons within the second interfacialcatalyzed region such that the rate of dissolution of the sequestrationagent can be maximized independent of mineral undersaturation at or nearequilibrium. In particular, in many embodiments it has been found thatthe addition of a suitable catalyst, such as, for example, carbonicanhydrase if delivered within a first catalysis region surrounding thesecond interfacial catalysis region (e.g., in some embodiments definedby the laminar boundary layer of the sequestration agent), can operateto equilibrate pools of CO_(2(aq)) and protons (e.g., H⁺ and H₂CO₃) anddrive dissolution rates of the sequestration agent up to 3 orders ofmagnitude faster than natural thermodynamics and kinetics allow for afixed level of mineral undersaturation. Accordingly, embodiments ofapparatus and method provides systems for introducing a catalyst, suchas, for example, carbonic anhydrase to a solution containing dissolvedCO₂ in a controlled manner at the interfacial catalysis region (660)defined at the interface between the seawater solution and the CaCO₃sequestration agent to greatly enhance the CaCO₃ dissolution kinetics(k_(seq)).

In accordance with embodiments, the formation of a controlled catalyzedregion where relatively high concentrations of protons are present inclose proximity to a sequestration agent (such as, for example, calciumcarbonate) also decreases reactant transport time. Reactant transport isanother slow step in the reaction process. By decreasing reactanttransport through, for example, water protolysis and hydrating CO₂ inthe presence of a sequestration agent, the two slow steps of thecarbonate dissolution are decreased, and the reaction rate is greatlyincreased. Thus, in many embodiments the system and method requires theformation of an aqueous solution containing at least water and CO₂ andthe controlled introduction of this aqueous CO₂ solution into a firstengineered catalyzed region containing at least a catalyst suitable forcatalyzing the production of protons from the protolysis of water and/orthe hydration of aqueous CO₂ to carbonic acid. The first catalyzedregion in turn encompasses a second interfacial catalyzed regionproximal to a source of a suitable sequestration agent, such as, forexample, a carbonate. By engineering the first catalyzed region andsecond catalyzed region such that the protons can be delivered betweenthe regions (i.e., from the first catalyzed region to the secondinterfacial catalyzed region) a virtuous cycle of CO₂ hydration,carbonate dissolution and diffusion of these reactants together isformed (630).

Engineered CO₂ sequestration carried out as described provides forgreatly enhanced sequestration rates not observed elsewhere or possiblewith other methodologies because the slow step of hydration is enhancedthrough catalysis and the requirements of reactant transport are alsominimized by carrying out both reactions concurrently in the same vesselin a controlled manner within the catalyzed regions. In such embodimentsthe system output (670) is the product of the dissolution reaction,e.g., in the case of a CaCO₃ sequestration agent, calcium andbicarbonate ions (Ca⁺² and HCO₃ ⁻).

Turning to the sequestration agent, in some embodiments thesequestration agent may be a mineral material such as for example,carbonates (e.g., metal carbonates), silicates, etc. Suitable calciumcarbonates may include any number of such mineral carbonate species,such as, for example, calcite, aragonite, dolomite, vaterite, etc. Invarious embodiments, one or more such sequestering agents may be used incombination. Accordingly, many embodiments use metal carbonatesequestration agents, while some embodiments use non-carbonatesequestration agents such as for example silicate minerals, and in somefurther embodiments, sequestration agents include various mineraladmixtures. Finally, in yet other embodiments, sequestration agents area combination of carbonate and non-carbonate solids.

Accordingly, although a calcium carbonate sequestration agent isdiscussed above, the choice of sequestration agent or agents may be madebased on their solubility characteristics to allow for the furthercontrol of the sequestration potential of the controlled catalysisprocess and apparatus. More soluble minerals will leave a mixture withless mineral and higher carbonate ion concentration at equilibrium. Inturn, a higher carbonate ion concentration is equivalent to a greatertotal neutralization, and thus greater potential CO₂ storage. As shownin FIG. 6, calcite is the least soluble mineral. Aragonite is moresoluble than calcite by about a factor of two, and vaterite is even moresoluble (i.e., by a factor of about five). Meanwhile dolomite spans awide range of solubility, and can be much less soluble than calcite insome forms, while disordered dolomite is more soluble than botharagonite and calcite. As further shown in FIG. 6, magnesium has astrong influence on the solubility of calcium carbonate. Indeed, in theplot it can be observed that calcite solubility increases as Mg contentincreases. Accordingly, in other embodiments the Mg content of calcitecan be tuned such that high-Mg content calcite can be used when a highlysoluble mineral is to be used.

Further, as previously discussed, the sequestration agent, regardless ofmaterial, may be incorporated into the system in many different forms,but in some embodiments, it may be optimally formed to maximize surfacearea exposed to the seawater-catalyst solution. For example, it may beformed into 1 mm³ cubes, or it may be crushed into smaller particles orshapes to maximize roughness to increase surface area. Alternatively,the sequestration agent may be incorporated into a large fluid path/highsurface area fluidized bed reactor having baffles or waffles or otherhigh surface area constructions. In general, greater sequestration agentsurface area allows for greater reactant diffusion, more dissolution andtherefore a higher rate of carbon sequestration, as will be understoodby those skilled in the art.

In some embodiments, systems, methods and apparatus are provided inwhich the catalysis of the dissolution and sequestration are diffusionlimited. In such embodiments, where the limit of catalysis to drive thereaction rate is reached, the only remaining limit on the capture andsequestration of carbon is in the ability of diffusion to deliver newreactants from the first catalysis region to the second interfacialcatalysis region at the sequestration agent surface, which can beconceptualized as the diffusive reaction flux. A diffusive reaction fluxcan be calculated from the gradient of products and reactants across aboundary layer of known thickness, and is defined as:

${{Flux} = {D\frac{d\lbrack C\rbrack}{dx}}},$where d[C] is the difference in concentration of reaction products inthe interfacial catalysis region near the sequestration agent surface(in this case presented as a carbonate) and in the bulk solution, dx isthe thickness of the boundary layer, and D is the molecular diffusioncoefficient of the carbonate ion. In some embodiments of this system,the term d[C]=[CO₃ ²⁻]_(sat)−[CO₃ ²⁻]_(bulk), where [CO₃ ²⁻]_(sat) isthe solubility product (divided by [Ca⁺²]) at the mineral surface and[CO₃ ²⁻]_(bulk) is the concentration in bulk solution. A diffusivereaction boundary layer may act as a barrier for diffusive reactionflux. At reasonable stirring rates, the diffusive reaction boundarylayer may be on the order of 10 microns. In embodiments of the inventionreaching the diffusion limit, transport may be limited only by d_(x),the thickness of the boundary layer.

In short, for a solid sequestration agent such as a carbonate mineral,the diffusion limitation is typically conceptualized as a reactant fluxcalculated for transport of products away from the mineral surface. Incontrast, in embodiments of systems, methods and apparatus where thediffusion limit is reached, the reaction may be limited by transport ofreactants to the mineral surface. Accordingly, in many embodimentsmethods and systems are provided to increase transport of reactantswithin the catalyzed region to the sequestration agent, to improve theability of diffusion to deliver new reactants to the catalyst region(e.g., by creating the thinnest boundary layer possible between thefirst and second catalyzed regions and/or delivering the maximum amountof reactant from the first catalyzed region into the second interfacialcatalyzed region).

In additional embodiments, methods can be employed to produce thethinnest boundary layer possible between the first catalysis region andthe interfacial catalysis region. Such method may include increasing theconcentration gradient to deliver the maximum amount of the reactant tothe interfacial catalysis region. Such methods may also include mixingor stirring the reactants, or placing the reactants and sequestrationagent in proximity through a high-surface area construction such as afluidized bed reactor. In embodiments, systems, methods, and apparatusesfor carbon sequestration may further include varying the surface areaand conformation of the sequestration agent. It has been shown that thesurface area of the sequestration agent affects the rate at whichdissolution and CO₂ neutralization will occur, as smaller grains havelarger surface areas than larger grains per mass. FIG. 7 provides a dataplot illustrating the relationship between grain size (in diameter, inμm) and dissolution rate for a given degree of mineral undersaturation(FIG. 7). The curves indicate that dissolution rate scales with grainsize. Thus in various embodiments, smaller grain size may be desirableto drive dissolution. In some embodiments, considerations regardinggrain size involve weighing several factors, including but not limitedto the energy and cost of crushing the sequestration agent material foruse in a reactor balanced with how rapidly that sized material will needto be replenished and how rapidly CO₂ sequestration can occur. In avariety of embodiments, particles of sequestration agent may be milli-or micrometer sized. Hence, material grain size is an adjustable andcontrollable parameter in driving CO₂ dissolution. The surface of thesequestration agent may also be roughened to increase the surface areaof the interfacial catalysis region and thereby maximize exposure of thefirst catalysis region to the second interfacial catalysis region.

As discussed above, various non-carbonate sequestration agents may beused in systems, methods, and apparatuses for CO₂ sequestration with theaddition of siderophore-like compounds or other catalytic compounds(such as carbonic anhydrase). In some embodiments, non-carbonatesequestration agents are silicate minerals. Weathering reactions on theearth's surface sequester CO₂ in the generalized reaction in whichsilicate minerals are altered to bicarbonate, a cation and dissolvedsilica.2CO ₂+3H ₂ O+CaSiO₃↔2HCO ₃ ⁻ +Si(OH)₄The incongruous weathering of Si-bearing minerals results inprogressively slower dissolution kinetics with time in a reactor,however the agitation of reactor grains and rapid kinetics ascatalytically enabled with the addition of siderophore-like compounds orother compounds (such as carbonic anhydrase) will enhance mineraldissolution and CO₂ sequestration. Accordingly, various mineraladmixtures including non-carbonate minerals may be used in variousembodiments.

In systems, methods, and apparatuses for carbon sequestration,temperature control will depend on the desired kinetic and thermodynamiceffects. For example, calcite becomes less thermodynamically soluble astemperature increases (FIG. 8), but more soluble as temperatureincreases due to kinetics. FIG. 8 depicts data plots illustratingcalcite solubility as a function of temperature and pressure. In theseplots, the curves demonstrate decreased solubility with increasedtemperature and increased solubility with increased pressure of CO₂(FIG. 8). Accordingly, in certain embodiments, such as those usingcalcite as a sequestration agent, it may be necessary to balancethermodynamic and kinetic processes in determining the appropriatedissolution temperature. In view of these competing concerns, which mayarise as a result of a chosen sequestration agent, temperature mayoptionally be determined based on either increased solubility to thedetriment of dissolution rate or increased dissolution rate to thedetriment of solubility.

In some embodiments of the invention, contact between the sequestrationagent and surface poisoning ions is curtailed via the addition ofcondition agents. The inhibitory effect of surface ions like phosphatehas been proven and is well known. For calcite, surface poisoning byphosphate ions is a function of at least: (1) the concentration ofphosphate ion in the solution, and (2) the available surface area ofcalcite that phosphate can adsorb to. FIG. 9 illustrates the inhibitoryeffect of surface poisoning ions in comparing the dissolution rate ofcalcite as a function of saturation states and phosphate concentration.FIG. 9 indicates that, below a threshold (in some embodiments anundersaturation of 0.2 and in some cases 0.3), lowering the phosphateconcentration increases the dissolution rate close to equilibrium. Thus,the data plot in FIG. 9 is at least also a function of the amount ofcalcite and the surface area of the available calcite, and demonstratesthat lowering the amount of adsorbed inhibitor per square meter ofcalcite surface area increases the reaction rate. To enhance CO₂sequestration, various embodiments of systems, methods, and apparatusesintroduce conditioning agents to lower the amount of adsorbed inhibitor,or surface poisoning ions in contact with the sequestration agentsurface(s).

Embodiments of the invention may also be controlled to operate atvarious pressures. In various embodiments, for many sequestration agentsincreases in pressure are proportionate to increases in solubility. Forexample, it is well known that pressure increases CaCO₃ solubility; anincrease from ambient surface pressure to 400 bar increases solubilityby a factor of two (FIG. 10) Under pressure, the kinetics of CaCO₃dissolution is effectively greater for a given saturation state ascompared to dissolution kinetics for the same mineral saturation stateat ambient pressure (FIG. 10B). Thus, dissolution under pressure bothdrives a faster dissolution rate due to greater solubility (FIG. 10A)and also increases the kinetic (rate) of dissolution (FIG. 10B). Higherpressure scales to higher dissolution rate, with a greater effectfarther from equilibrium. Accordingly, in some embodiments, rates ofdissolution are controlled using pressure as one of the adjustableparameters. In many such embodiments, a pressure of at least 500 psi isused, in other embodiments at least 1000 psi, in still other embodimentsat least 1500 psi, and in yet other embodiments at least 2000 psi.

Embodiments of the invention may use various aqueous solutions. Salinityof these solutions can affect the solubility of various sequesteringagents. For example, the solubility of CaCO₃ is greater in salty water(given NaCl as the major salt in solution) compared to fresh water. Inresponse to increased solution salt content, corresponding changes tosolubility are non-linear and there is a maximum in solubility based onconcentration of NaCl (FIG. 11). Thus, in some embodiments, the use ofbrine solutions may be used to yield faster dissolution than would occurin fresh waters. Accordingly, in some embodiments the concentration ofNaCl is form at 1-6 M NaCl, in other embodiments from at 1-5 M NaCl, instill other embodiments from at 1-4 M NaCl, and in yet other embodimentsfrom at 2-3 M NaCl.

Although the discussion to this point has focused on a carbonicanhydrase catalyst in various embodiments of carbon sequestrationmethods, one or more catalysts may be presented in a variety of formssufficient to create a zone of catalysis. Accordingly, in manyembodiments any catalyst suitable for: (1) increasing the protolysis ofwater to form OH⁻ and H⁺, and/or (2) increasing the local activity ofhydroxide ion (OH⁻), and/or (3) increasing the local activity ofhydrogen ion (H⁺) and/or, (4) increasing the physical separation betweenOH⁻ and H⁺, all without a change in the bulk pH of solution may beutilized. Exemplary embodiments of such catalysts may include, forexample, carbonic anhydrase, carbonic anhydrase analogues and ormaterials with carbonic anhydrase active sites, and nickel, such asnickel nanoparticles, among others. In other embodiments the catalystcan be fixed to surfaces within a reactor vessel or fixed to surfaceswhich are then fed into a reactor vessel in some embodiments. In variousembodiments, catalyst is dissolved. In further embodiments, dissolvedcatalyst in an effluent stream is recovered using enzyme separationtechniques. In yet other embodiments, to separate enzyme, enzymefiltration is used. Enzyme filtration can be accomplished using anycombination of polyethersulfones (PES), cellulose acetate (CA),sulfonated polyethersulfones (SPES), or polyvinylidene fluoride (PVDF)nanofiltration or ultrafiltration membranes using membrane layers,membrane pore, or hollow fibers. In further embodiments, membranefiltration is set up to periodically reverse flow direction relative tothe filtration membrane or utilize non fouling membranes. Anycombination of these techniques for providing and retaining catalyst canbe used in additional techniques.

Although specific systems, methods, and processes for carbonsequestration by utilizing catalysis schemes in proximity to asequestration agent are discussed above with respect to FIGS. 2 to 11,any of a variety of systems, methods, and/or processes for performingcarbon sequestration utilizing a catalysis scheme in combination with asequestration agent as appropriate to the requirements of a specificapplication can be utilized in accordance with embodiments of theinvention. Thus, for example, in many embodiments a variety of reactantsmay be used, a variety of sequestration agents may be used, includingbut not limited to carbonate compounds, e.g. calcium carbonate, andnon-carbonate compounds used alone or in combination. In otherembodiments various catalysis schemes may be utilized to provide highconcentrations of hydrated CO₂ in proximity with a sequestration agent.Additionally, the aqueous solution is not limited to seawater asdescribed above, but may be briny or freshwater. In several embodiments,solutions may involve, but are not limited to, aqueous solutions ofneutral, circum-neutral, or acidic pH. Furthermore, for severalembodiments, it should be clear that [CO_(2(aq))] may vary. Although asingle reaction is described in relation to the process and methods, inother embodiments, the method steps may be repeated such that seawaterstill containing undissolved CO₂ may be further hydrated to morethoroughly sequester the carbon. Embodiments, systems, methods andapparatuses for carbon sequestration may also involve the adjustment ofseveral parameters. In embodiments, these parameters include at leastthe surface area, conformation, or type of sequestration agent, exposureof sequestration agent to surface poisoning ions, pressure, temperature,and salinity. Methods to tune the mineral undersaturation and diffusioncoefficient may also include altering the temperature, pressure, and/orsalinity within one or both of the catalyzed regions. Alternatively, thetemperature and/or pressure of the system can also be controlled toincrease dissolution of the saturation agent at a fixed mineralundersaturation.

Comparison With Conventional Sequestration Systems

Other sequestration strategies use carbonate dissolution, but thesediffer from the methods and embodiments described in some importantaspects. Conventional CO₂ hydration systems typically describe atwo-step process involving separate hydration and dissolution steps, asshown in FIG. 12. In the first step, hydration, inputs are gaseous CO₂(905) and water (910) and outputs are a CO₂ depleted gas stream (915),CO_(2(g)) liberated from a hydrated solution (degassed H₂CO₃ and CO₂)(920), and an aqueous solution containing a mixture H₂CO₃* (925)comprising H₂CO₃ and orders of magnitude greater amounts of CO_(2(aq)).In the sequestration step, the solution containing H₂CO₃* (925) isreacted with a sequestration agent (930) such as a metal carbonate toyield a metal and bicarbonate product (930). In conventional systems oneof the primary objectives of the first step (hydration) of this processis to increase the amount of H₂CO₃* in solution by dissolving highconcentrations of CO₂ into solution, thus increasing carbonic acid insolution. In such systems it is generally preferable that the carbonicacid solution be as low in pH as allowed by the operating temperature,the incoming gas stream's pCO₂, the water volume with which CO₂ ishydrated, and the effects of chemical additives (if any). The CO₂ input,or gas stream, can be recirculated for further CO₂ removal. In thesecond step of the sequestration process, the carbonic acid is reactedwith metal carbonate to yield a metal and bicarbonate product (930).

In these conventional methods of sequestration, emphasis is on the firststep of the dissolution reaction—the hydration step. Such methodsrequire that as much CO₂ as possible be dissolved into solution (toincrease CO_(2(aq))) to achieve the highest possible concentration ofcarbonic acid. However, a problem with this, and emphasis on increasingH₂CO₃* to sequester carbon in general, is that there are small amountsof H₂CO₃ relative to CO_(2(aq)) in such solutions, and further there isa strong tendency for CO_(2(aq)) to degas from solution.

In many embodiments of the systems, methods, and processes, theseproblems are resolved by a novel method of catalysis whereby acontrolled catalyzed region rich in protons (e.g., H⁺ and H₂CO₃ isformed in close proximity to the sequestration agent (e.g., carbonateminerals). As described above, such systems, methods, and processesobviate the potential problems associated with the transport ofdissolved CO₂ and its degassing because large amounts of CO₂ are, notforced into solution to increase H₂CO₃* levels. As shown in FIGS. 2 to11, for embodiments of the systems and methods utilizing the catalyzedcarbonate reaction region, no additional CO₂ need be dissolved incircum-neutral pH solutions for seawater during hydration. Degassing isfurther minimized because reactant transport is minimal since within thecatalyzed region the hydrated CO₂ reacts almost immediately withcarbonate after the protons are formed. Accordingly, embodiments ofsystems and methods for carbon sequestration eliminate problemsassociated with CO₂ degassing by decreasing the need for high levels ofgaseous CO₂, and also minimizing reactant transport. The result is acarbon sequestration method that operates at near equilibrium and ismuch faster than processes that require far from equilibrium mineralundersaturation to operate at comparable rates.

Catalyzed Sequestration Apparatus Embodiments

Although systems and methods of carbon sequestration in accordance withembodiments have been described, it should be understood that apparatusembodying these novel systems and methods are also contemplated. FIGS.13A and 13B provide exemplary sequestration systems as applied in manypossible apparatus embodiments.

As shown in FIG. 13A, various embodiments of the invention can include(but are not limited to): a reactor vessel (705), a CO₂ input (710), anaqueous solution input (715), a sequestration agent input (720), ahydrating catalyst input (725) and/or a bicarbonate solution outputstream (745), as illustrated in exemplary apparatus 700. In theseembodiments, the various inputs, outputs and reactants are arranged suchthat at least the sequestration agent, catalyst and CO₂ solution arebrought into reacting proximity to form two controlled catalysis regions(735 and 735′) or zones within the reactant vessel.

In many embodiments, the first catalysis region (735) is formed in aregion surrounding the sequestration agent where the CO_(2(aq)) solutionand catalyst are introduced together. This first catalysis region, inturn, encompasses a second interfacial catalysis region or zone formedaround at least the interfacial region of the sequestration agent (e.g.,at the laminar boundary layer of the sequestration agent) such that acatalytic cycle is formed whereby the rate of proton replenishment from(CO₂ hydration and water hydrolysis) in solution at the interfacialregion are enhanced. In some embodiments the interfacial catalysisregion is defined as the volume within the reaction vessel at leastwithin the diffusive reaction boundary layer of the sequestration agent.In other embodiments, the interfacial catalysis region may be defined bythe laminar boundary layer of the sequestration agent, thereby allowingfor an increase in the concentration of protons in the solution bulk,which can serve to increase the dissolution of the sequestration agent(e.g., carbonate ion) at mineral undersaturations near or atequilibrium. In many embodiments, the apparatus produces a sequestrationagent dissolution rate within the controlled catalysis region greaterthan the sequestration agent dissolution rate of an uncatalyzed systemat the same solution undersaturation. In many embodiments, the apparatusproduces a sequestration agent dissolution rate within the controlledcatalysis region at least one order of magnitude greater than thesequestration agent dissolution rate of an uncatalyzed system at thesame mineral undersaturation. In other embodiments, the sequestrationagent dissolution rate within the controlled catalysis region is atleast two orders of magnitude greater than the sequestration agentdissolution rate of an uncatalyzed system at the same mineralundersaturation. In still other embodiments, the sequestration agentdissolution rate within the controlled catalysis region is at leastthree orders of magnitude greater than the sequestration agentdissolution rate of an uncatalyzed system at the same mineralundersaturation.

In many embodiments, input feed streams flow into the reactor at variouspoints and flow out of the reactor at various points such that thecatalysis regions are formed and preserved throughout the sequestrationprocess. Still, in other embodiments, the input and output streams maybe calibrated to operate at steady state. In yet other embodiments, thefeed streams do not operate at steady state. In other embodiments thecatalysis zones are formed and preserved by separate structures withinthe reaction vessel that defined the controlled catalysis zones (e.g.,permeable vessels within the larger reactor vessel, fluidized beds, flowpaths, waffle or baffle constructions, etc).

In some embodiments, the reactor vessel has sufficient volume such thatthe first catalysis region forms about a second interfacial catalysisregion at the CO_(2(aq)) solution-sequestration agent interface (e.g.,at the laminar boundary layer of the sequestration agent). Catalyst maybe added at various points in the reactor to enhance the formation ofthe catalysis zone at the CO_(2(aq)) solution-sequestration agentinterface. In additional embodiments, the reactor may be of a sizeand/or volume such that a mixing zone forms wherein various inputs maythemselves mix together and additionally mix with the sequesteringagent. It will be understood that such a mixing region may or may notextend outside the first and/or second catalysis zone such thatreactants are carried into at least the first and/or second catalysiszone for sequestration as a result of the mixing. Additionally, thereactor may be of any size deemed sufficient by one skilled in the artto accommodate reactor inputs, sequestration agent, and reactor outputs.Various embodiments of the invention involve a reactor vessel of a shapedetermined by one skilled in the art to be suitable for the describedembodiments of the invention.

In some embodiments, the reactor vessel will have at least one input,which may involve a mixture comprising at least CO₂, aqueous solution,and/or catalyst, and at least one output, which involves at least abicarbonate-enriched solution.

Various embodiments of the invention will have different configurationsof input streams, such that the flows combine in the reactor to yieldcontrolled regions of catalysis. In FIG. 13A, the embodiments of theexemplary apparatus 700 shown have at least five input streams,involving (but not limited to): CO₂ (710), H₂O (715), CaCO₃ (720),chemical additives (i.e., catalyst and anti-poisoning materials), andrecycled components (740). Calcium carbonate (CaCO₃) (720) is used as asequestration agent in the embodiments described, but it will beunderstood that other sequestration agents may be substituted.Furthermore, chemical additives are described as at least a catalyst,such as carbonic anhydrase, but one skilled in the art will recognizethat other catalysts may be substituted. Thus, embodiments of theinvention may have unique inputs for each reactant feed stream, i.e.,separate streams for CO₂, aqueous solution, catalyst, and sequestrationagent. Alternatively, in other embodiments, it will be understood thatthere may be fewer input streams, particularly where multiple reactantsare combined into a single stream. For example, CO₂ and aqueous solutionmay be combined into a single stream, and this may be also furthercombined with sequestration agent. A second input stream may containcatalyst. The two streams can be combined in a reactor such that the twocatalysis regions form about the solution-sequestration agent interface.One skilled in the art will recognize that one, two, or three inputstreams may be used and that in various reactants may be combined intosingle input streams.

The location of input and output streams may be at various locations inthe reactor. Reactants and catalyst may flow into the reactor throughinputs located at points to at least partially ensure the formation offirst catalysis zone encompassing a second interfacial catalysis regionat the interfacial region around the sequestration agent. Outputs (745)may be located at points on the reactor to remove bicarbonate productwhile maintaining the presence of the catalysis regions (735)surrounding the sequestration agent.

Inside the reactor, inputs may be combined in a variety of ways tofacilitate the formation of the catalysis regions, as described above.As is shown in the exemplary embodiment described in FIG. 13A, inputsincluding (but not limited to) CO₂ (710), aqueous solution (715),chemical additives, including catalyst (725), and/or sequestration agent(720) may be combined in a reactor mixing zone, resulting in theformation of a first region of catalysis (735) and a second interfacialcatalysis region (735′) surrounded by the first catalysis region anddisposed at the interface of the sequestration agent (CaCO₃) bed.According to embodiments, sequestration agent may be present in variousconformations, i.e. millimeter scale particles, to increase catalysiszone surface area. In a variety of embodiments, CO₂ may be combined withat least aqueous solution inside the reactor. For example, in someembodiments the catalysis region (735) may be formed, for example, as aseparate permeable vessel within the larger reactor vessel (705). Insuch an embodiment the catalysis region may have inlets along itssurface sized such that the catalyst and sequestration agent materialsare confined therein, but wherein the other elements in the reaction(e.g., aqueous solution and CO₂) are allowed to freely intermingletherewith or are move therethrough in a pressurized stream thus creatingand enforcing the first and second catalysis regions.

In the reactor, in addition to catalysis (discussed in more detailbelow) CO₂ hydration may be enhanced by techniques involving (but notlimited to): increasing the surface area of the aqueous solution incontact with a given volume of gaseous CO₂ and/or bubbling, increasingthe pCO₂ of the income gas stream, increasing pressure of the CO₂ gasstream and increasing CO₂ solubility by decreasing temperature. In otherembodiments, CO₂ and aqueous solution may be combined prior to reachingthe reactor. For example, CO_(2(aq)) may already be present in somesolutions, i.e. seawater, or it may be mixed in a mixing vessel prior toentering the reactor vessel.

The reactor may be maintained at various pressures to modify thedissolution rate of the sequestration agent in accordance withembodiments of this invention as set forth as determined by one withskill in the art. In some embodiments, reactor pressure may be increasedby the injection of gas (for example, CO₂) under pressure or injectionof solution (for example, water) under pressure. In further embodiments,pressure is released through check valves. In yet other embodiments,pressure is released by regulating the outflow pressure relative to theinflow pressure.

Catalyst can be added in quantities sufficient such that a firstcatalysis zone forms to encompass a second interfacial catalysis regionat the interface of the sequestration agent and solution. Theconcentration of catalyst in such embodiments can be tuned to one orboth: 1) maximize the replenishment of bound protons from H₂CO₃ producedfrom the CO_(2(aq)) reservoir of the reaction vessel; and 2) maximizethe production of free protons from the protolysis of water. In someembodiments, the solution may be mixed over a bed of sequestrationagent, and/or with particles of sequestration agent to increase surfacearea and reduce the boundary layer between the first and secondcatalysis regions. Where the diffusion limit is reached, catalyst may beadded such that there is a standing stock of carbonic acid in the firstcatalysis region such that a maximum amount of reactant is deliveredinto the second interfacial catalysis region.

The sequestration agent used in the reactor may involve a variety ofphysical conformations. To enhance sequestration, various embodimentsmay utilize conformations of sequestration agent that have increasedsurface area. In some embodiments, considerations regarding grain sizeinvolve weighing several factors, including but not limited to theenergy and cost of crushing sequestration agent for use in a reactorbalanced with knowledge of how rapidly that sized material will need tobe replenished and how rapidly CO₂ sequestration can occur. In a varietyof embodiments, particles of sequestration agent may be milli- ormicrometer sized.

Particles may be fed into the reactor as needed. Or alternatively, inother embodiments particles may be replaced in batches. In yet otherembodiments, sequestration agent may exist as a bed within the reactor.A zone of catalysis can form over the surface of the bed, as catalystand CO_(2(aq)) solution are distributed over it. Sequestration agentmaterial may be of varying sizes and roughness to enhance surface area.Alternatively, the sequestration agent may be incorporated into a largefluid path/high surface area fluidized bed reactor having baffles orwaffles or other high surface area constructions.

The reactor may be maintained at a temperature deemed by one skilled inthe art appropriate for the sequestration process to proceed anddissolution rate and diffusion coefficients of the sequestration agentto be optimized.

Fresh or saline water containing CO₂ may be fed into the reactor. Insome embodiments, the water is freshwater. In many embodiments inletwater is seawater, or another saline solution. In some embodiments abrine solution will yield greater rates of dissolution.

The reactor may also be maintained at neutral, circum-neutral, or acidicpH, or at pH levels such that the sequestration process proceeds inaccordance with embodiments of this invention as set forth as determinedby one with skill in the art. In other embodiments, the reactor mayinvolve a stirrer to decrease the boundary layer surroundingsequestering agent materials, particularly when the sequestration ratereaches the diffusion limit.

In many embodiments, the reactor may have monitors (750) at variouspoints to monitor various parameters, including reactant, catalyst,pressure, salinity, surface poisoning ions, product, pH, andtemperature. In various embodiments, monitors may be located at variousinput and output streams at points where the stream flows into thereactor. Other embodiments may involve monitors located inside thereactor. More specifically, monitors inside the reactor may be placed todetect and measure zones of catalysis and sequestration.

In various embodiments of the invention, input streams feed at least onestream of CO₂, aqueous solution, and catalyst into the reactor. In someembodiments, an additional sequestration agent feed may replace depletedsequestration agent inside the reactor vessel. In other embodiments, thesequestration agent may be replenished to the reactor in batches. Inputstreams may comprise combinations of reactants that are combined priorto reaching the reactor. Various embodiments can involve for example(but are not limited to) one input stream comprising at least CO₂,aqueous solution, and sequestration agent combined and an additionalinput stream comprising at least catalyst sufficient to ensure theformation of a zone of catalytic activity. Some embodiments mayadditionally include a mixing vessel to combine input streams beforethey reach the reactor. For example, this mixing vessel may dissolvegaseous CO₂ into aqueous solution before addition of the catalyst.Additionally, sequestration agent may be fed into the reactor in aseparate stream. Some of these mixed-input streams may comprisecomponents recycled from effluent streams including at least one ofaqueous solution, sequestration agent and catalyst (740).

In some embodiments of the invention, outputs (745) of the reactor mayinclude at least a bicarbonate-enriched solution. Additionally, outputsmay involve (but are not limited to): excess aqueous solution, CO₂,catalyst and/or sequestration agent. In many embodiments, outputs mayleave the reactor in at least one stream, so long as there is a zone ofcatalysis at the interfacial region between the solution and thesequestration agent. Additionally, in various embodiments, outputs mayleave the reactor at various points, such that the zone of catalysisremains at the interfacial region is maintained. Placement and/or flowof output streams may also be designed to ensure that the zone ofcatalysis remains at the interfacial region is maintained. The abovevariables may also be optimized to maximize the zone of catalysis.

Optionally, some embodiments may have a filter (755), from which variouscomponents of the output stream (745) can be extracted, retained, orrecycled, resulting in an effluent stream (730).

Many embodiments employ various mechanisms for retaining sequestrationagent at the reaction site (755) (FIG. 13A). In some embodiments,effluent water is filtered using a particle filtration system (FIG. 14).In additional embodiments, a settling chamber is optionally added wherereactant can settle to the bottom and effluent, solid-free water can besipped or discharged off the top of the chamber. The settling chambercan be the main reaction vessel or a secondary reaction chamber. In yetother embodiments, a combination of mechanism for retainingsequestration agent may be used.

Optionally, some embodiments may have one or more reactors, which arefed from a first reactor, to hydrate and sequester any CO_(2(aq))remaining in solution. As an illustration of an exemplary embodiment,FIG. 13B shows a second reactor (780) located downstream from the firstreactor with inputs that may primarily include outputs (745) from thefirst reactor, particularly aqueous solution and undissolved CO₂ withcatalyst. This primary flow leaving the first reactor feeds a secondreactor, which is parameterized separately (782) in order to completelyneutralize or degas any residual CO₂. In other embodiments, additionalinputs to the second reactor may include catalyst (784), i.e., carbonicanhydrase as shown in FIG. 13B or other catalysts recognized asappropriate by one having skill in the art. In other embodiments,sequestration agent may be present in batches, e.g., a carbonate bed(786) as shown in FIG. 13B, or sequestration agent may be replenished ina steady state or as deemed necessary through optional monitoring. Inmany embodiments, the secondary reaction taking place in the secondreactor may provide a CO₂—neutral, alkaline solution as effluent. Theresulting effluent can be mixed with ambient environmental fluids, suchas seawater. In some embodiments employing a two-stage reactor where thesequestration agent is retrograde soluble, the temperature may be highin a first stage and lowered in a second stage such that the firstreactor has the fastest dissolution rate, but the second reactorincreases the total amount of carbonate dissolved over a longer time. Inembodiments using such two vessel arrangements one or both of the firstor second vessels may include the two-catalysis region constructsdescribed above. The size of the two reactors may vary according to thedesired reaction time.

In many embodiments of an apparatus, the apparatus controls andcontinually adjusts mineral undersaturation in the reactor device usinga feedback loop (790). In a feedback loop, a control system may monitorand adjust, for example, the input of various parameters, particularlycatalyst and CO₂ into the reactor at circum-neutral pH (between 6.5 and7), as shown in FIGS. 13A & 13B. In some embodiments, the control systemoperates to ensure that the apparatus operates at the maximum, catalyzeddissolution rate, neutralizing as much CO₂ as possible per unit reactiontime. In a variety of embodiments, the control system may processmonitored input parameters and may adjust other process parametersappropriately.

In many embodiments, the control system (760) monitors the process fromvarious monitoring points. Some embodiments of the invention may involvemonitors at various points in the reactor apparatus. Monitors may detectparameters (765) including but not limited to pH, temperature, pressure,salinity, surface poisoning ions, pCO₂, TCO₂, flow rate, and/or reactantinput, including sequestration agent, and catalyst input. Variousembodiments have monitors inside the reactor (770). Inside the reactor,monitors may in some instances be located at input and/or outputstreams. Alternatively, monitors may also be placed near the catalyticzone (e.g., within the interfacial region of the sequestration agent).

In various embodiments, catalyst retention strategies may be employedsuch that catalyst is appropriately provided in the carbon sequestrationprocess and retained. Retention of catalyst minimizes potentialenvironmental impact and reduces catalyst cost. Reactor embodimentsutilize various methods of catalyst retention (FIG. 14). In some reactorembodiments, catalyst is fixed to the surface of free-floating solids(i.e., beads) surface, and these solids are retained using the samemethods as for the retention of reactant materials (805). In yet somemore embodiments, catalyst is fixed to surfaces within the reactorvessel. Surfaces to which catalyst can be fixed includes but are notlimited to reactor walls, membranes, rods, sheets, or any object thatcan be fixed into the reactor chamber. In additional embodiments,dissolved catalyst can be separated from a solid-free effluent streamand retained using an enzyme separation technique and returning thecatalyst rich stream to the reaction vessel. For enzyme filtration, anycombination of PES, CA, SPES, PVDF or equivalent nanofiltration orultrafiltration membranes (810) using membrane layers, membrane pores,or hollow fibers may be employed. In various embodiments, membranefiltration is set up to periodically reverse flow direction relative tothe filtration membrane or utilize non-fouling membranes. In additionalembodiments, catalyst may be replenished using an organism thatexpresses carbonic anhydrase. Additionally, a variety of valves may beused to control the rate and direction of output and effluent flow.Additional other embodiments use engineer organisms to produce catalyst.Organisms can be located within or outside the reaction vessel. In someembodiments, catalyst retention strategies can be combined.

In short, several embodiments of the invention involve a reactor withinwhich CO₂, aqueous solution, sequestration agent, and catalyst arecombined such that a first controlled catalysis region is formed toencompass a second interfacial catalysis region proximal (e.g., a thelaminar boundary layer) of the surface of the sequestration agent toenhance rates of carbonate sequestration. One skilled in the art willrecognize that the illustration is not a complete description of theinvention and that inputs and outputs may be combined in a variety ofways in the second reactor to create such a catalysis zone.

Data Collection

The following provides information about the methods used to conduct theexemplary studies that produced the data provided in FIG. 3 showing theunexpectedly improved sequestration rates obtained using systems andprocesses in accordance with embodiments. It will be understood thatthis data and the accompanying examples are only provided asillustration and are not meant to limit the scope of the systems,methods and apparatus described throughout the disclosure.

Methods and Materials

In the observations provided with this disclosure in relation to FIG. 3particularly, high precision stable isotope measurements, ubiquitousgeochemistry, and chemical oceanography were used. The experimentmethodology exploits the stable isotope of carbon ¹³C as a direct tracerof mass transfer from mineral to solution. In general, calciumcarbonates enriched in ¹³C are placed in a closed system ofundersaturated seawater. Then, the evolving δ¹³C of this seawater ismeasured over time by discrete sampling, thus obtaining curves of molesdissolved over time.

Calcium carbonates enriched in ¹³C, while rather inexpensive andplentiful, are not ideal due to its sintered nature. A well-formedmaterial that could be manipulated as an inorganic solid in a range ofgrain sizes is needed. To this end, calcium carbonates were grown in thelaboratory, using a gel-diffusion method first described by Nickl, H.J., Henisch, H. K, 1969. Journal of the Electrochemical Society SolidState Science 116 (9), 1258-1260, the disclosure of which isincorporated herein by reference. In this method, a glass U-shaped tubeis filled with 50 mL hydrous gel (in this case, pH-adjusted sodiummetasilicate (0.17 M), separating 30 mL reservoirs of CaCl₂ and Na₂¹³CO₃ (both 0.15 M) in each arm of the tube. The ends of the tube aresealed using Parafilm and rubber stoppers. In the process, nucleation ofcalcium carbonate crystals is limited by diffusion and the gel porespacing, allowing for slow growth of large grains. The grains areharvested after 3-6 months of reaction time by pouring off the spentreservoir solutions followed by physical break-up, sonication, anddecantation of the less dense gel matrix from the calcium carbonategrains. The grains are then triply washed in DDW and dried at 60° C. Inthis study, data is presented from gel-grown calcite, dry-sieved toseveral size fractions. The degree of isotopic labeling is measuredusing the Picarro CRDS on small (0.2 mg) aliquots of material,pre-acidified, and measured using the AutoMate Liaison autosampler.

The carbonate saturation state was constrained using dissolved inorganiccarbon and alkalinity pairs. All experiments were performed in a Dicksonseawater reference material (poisoned with mercuric chloride). Seawateraliquots of 2-3 L are transferred via siphon to 5 L Supelco gastightfoil bags. Mineral undersaturation is achieved by titrating alkalinityvia injection of HCl (0.1 M) through the sampling port septum of thefoil bag. Thus, no DIC is lost during alkalinity titration. DIC doeschange slightly, but only due to dilution by the added HCl solution.

Alkalinity, determined by open-system Gran titration, is performed on acustom-built instrument. A Metrohm electrode connected to a MettlerToledo SevenCompact pH meter was used. The titrant (0.05-0.1 M HCl innatural seawater medium) is delivered by a Metrohm 876 Dosimat Plustitrator with a 5 mL burette. A filtered 16 mL seawater sample is placedin a 25° C. water bath. The sample is stirred and bubbled with airthroughout the measurement. The titration program controls the titrationfrom a Windows laptop. Alkalinity is determined using a nonlinearleast-squares approach as outlined in the Best Practices Guide (Dicksonet al., Pices Special Publication 3, IOCCP Report No. 8. (2007)_, thedisclosure of which is incorporated herein by reference). Dicksonstandard reference materials, as well as an in-house seawater alkalinitystandard, are run at the beginning and end of every session to ensureanalytical consistency and to monitor acid and electrode drift.Long-term alkalinity precision is about 2.5 μeq/kg. Long-term accuracyis about the same as precision; thus total alkalinity error over thelong-term should be on the order of 3 μmol/kg.

Dissolved Inorganic Carbon (DIC) and seawater δ¹³C are determined usinga modified Picarro cavity ring down spectrometer with an AutoMateLiaison autosampler. About 7 mL of filtered seawater is injected into anevacuated 10 mL AutoMate vial from a syringe through the rubber septumscrew-cap. The net sample weight is taken. The AutoMate acidifies thesesamples on-line using 10% phosphoric acid, and the resulting CO₂ iscarried in a nitrogen stream, through a Nafion desolvating line, to thePicarro Liaison sampling bags. The flow rate is set to 80±0.2 cubiccentimeters per minute (ccm) by a mass flow controller in between theautosampler and the Picarro. Drift in both DIC and δ¹³C are monitoredover the course of the run, and also over longer time periods. DICvalues are corrected to reference material values, and samples are bothblank-and standard-corrected. Since there are no available seawater δ¹³Creference materials, δ¹³C values are also normalized to a value of 1%(VPDB). Samples are corrected for instrumental drift using linearinterpolation between bracketing standards (at the beginning, middle andend of the run). Also, a negative correlation between water content and[¹²CO₂] was documented. Water content in samples was monitored, and awater correction was made as necessary as well. Drift is almost neverabove a few tenths of a permil, and resulting Picarro standardstypically have a standard deviation of under 0.1%. Replicate DIC and Alkanalyses were taken advantage of, and standard errors were used whencalculating experimental Ω.

Alkalinity and DIC pairs are then converted to saturation state usingCO2SYS run through MATLAB. The errors in Alkalinity and DIC arepropagated to Ω_(calcite) by a monte carlo approach: Alk-DIC pairs aresampled randomly from normal distributions with their associatedstandard errors as the standard deviations, and the resulting Ω valuesare averaged. Errors on Ω, calculated this way, are between 0.01 and0.04 units. Carbonic acid dissociation constants are taken from theDickson and Millero (Dickson & Millero. Deep-Sea Research 34, 1733-43(1987), the disclosure of which is incorporated herein by reference)refit of Mehrbach's data (Mehrbach, et al. Limnology and Oceanography18, 897-907 (1973), the disclosure of which is incorporated herein byreference). The calcite solubility data from Mucci (Mucci, Am. J. Sci.09; 283(7) (1983), the disclosure of which is incorporated herein byreference) are used for calculation of Ω in CO2SYS.

All dissolution rate data presented were obtained on the benchtop atambient temperature (20-22° C.). Several different materials forexperiments were evaluated, since excellent control on saturation statewas needed.

Experiments were performed in Supelco inert foil bags, which are stablefor DIC and do not bleed or remove alkalinity, and polycarbonatesampling ports were fabricated. These ports have a built-in filterhousing, such that sampled water is filtered through Nucleopore membranefilters (˜0.2 μm). The port is fitted onto the bag through a punchedhole, hand-tightened, and sealed with a Viton o-ring. Using this setup,both alkalinity and DIC blank experiments show no change over days toweek.

Data is obtained in the following way: Supelco bags are cut open, andthe sampling ports are fitted through the foil. Labeled material (3-5mg) is weighed out and poured into the foil bag. The open bag is thenheat-sealed twice. These bags are then evacuated to remove allheadspace. Undersaturated fill water is then siphoned from its largefoil reservoir into these experimental bags. First, about 50 grams issiphoned in, and grains are agitated and rinsed. This water is thenremoved through the sampling port via syringe and discarded. Then, about300 grams of fill water is siphoned in, the bag is weighed to obtain theexact mass of water added, and the experiment is considered started.Once the experiment has started, bags are placed on a shaker table at 60rpm. The shaking rate has been tested and it was found that at speedsabove 60 rpm, the dissolution rate is the same as the rate at 60 rpm.Below 60 rpm, rates slow significantly, presumably due to stagnation andthe formation of boundary layers around the grains. At each samplingpoint, the experimental bags are weighed. Samples are taken through thesampling port via a tygon tube attached to a plastic syringe. Thesyringe is washed with about 2 mL of the sampled water, and then a full7 mL sample is taken. This sample is injected through a 0.45 μm filterinto a pre-evacuated AutoMate vial for Picarro analysis, as describedabove. Initially, sampling occurs 2 or 3 times daily. As the experimentproceeds, however, sampling becomes more infrequent. Total experimentduration is 3-10 days. Since DIC and δ¹³ are measured simultaneously,DIC is monitored over the course of the run. Post-experiment alkalinitymeasurements are taken to check for alkalinity consistency.

Carbonic anhydrase solutions are prepared by dissolving carbonicanhydrase powder (Sigma Aldrich, CAS 9001-03-0) into deionized water andmaking serial dilutions. At high concentration (1 mg/mL), carbonicanhydrase solutions resemble those of soap, with a viscous surface andbubbles. Dissolution experiments are prepared as described above using70-100 μm Aldrich 13C-labeled calcite. Just before the first time pointis taken, a carbonic anhydrase solution is added to the experimental bagvia syringe through the sampling port. The bag is then mixed well andthe first time point is taken.

Effluent Characterization and Environmental Impact

A CO₂ neutralizing scheme will fail unless the reactant solution isabundant and leaves the scheme with no environmentally untenable impact.The systems, methods and apparatus detailed in the figures are based ona sequestration agent, such as carbonate solid reacting with CO₂ in aseawater medium, but could be applied with freshwater. The environmentalimplications of both scenarios are discussed below. In all cases thesystems provided according to embodiments provide apparatus andprocesses that produce an outflow effluent having a lower partialpressure of CO₂ than the partial pressure of CO₂ provided in the in-flowto the system.

Many power plants use seawater as a coolant in a single pass orrecycling mode. For example, withdrawal of seawater to cool a nuclearplant in the UK occurs at a rate of 8·10¹⁵ L/day. If there were 1000sequestration reactors as described in the embodiments, each operatingso as to neutralize a proportionate daily CO₂ emission, then eachreactor would need to dissolve 2.5·10¹¹ g of CaCO₃ per day. Assumingocean surface waters have an alkalinity of 2100 μeq/L, the reaction ofCaCO₃ described above would add alkalinity so as to increase ambientwater alkalinities by 0.03%, a very small increase. Cooling water flowat a much lower rate would likely enhance alkalinity by only a fewpercent in the effluent. As these calculations demonstrate, a very smallfraction of the cooling water may be shunted to a carbonate reactor siteand returned to the effluent flow with minimal impact on effluent waterquality.

Returning water to the surface ocean with slightly enhanced alkalinityis likely considered a trivial and non-threatening environmentaloutcome. For example, the upwelling rate of 3 m/day can occur in coastalregions for several months and over vast areas encompassing thousands ofkm of shoreline. Estimated coastal upwelling occurs at a rate of 1 Sv(1·10⁶ m³/sec=8.6 10¹³ L/day) per 1000 km of shoreline. Upwelling offcoastal California adds alkalinity to the surface of the ocean at a rateof 3·10⁵ μeq alkalinity/day (assuming 100 m water alkalinity of 2120 μMand surface alkalinity=2085 μM, Berelson, unpublished data). Embodimentsof some reactors (assuming there are 1000 reactors) would only add5×10¹² peq alkalinity/day to coastal ocean. The neutralization of aday's worth of global CO₂ emission, reacted with CaCO₃ and dumped in thecoastal ocean would add alkalinity to the ocean in an amount equivalentto the amount of alkalinity added during upwelling along 1000 km ofcoastline. In other words, the coastal ocean, along only 1000 km,upwells more alkalinity to the surface ocean in one day than one giantreactor plant could add to the ocean in one day. Based on estimates,there may need to be about 3 reactors per 1000 km of coastline, which isstill over 2 orders of magnitude below the natural flux of alkalinityfrom deep waters. Furthermore, the addition of alkalinity enrichedwaters to some coastal environments may be seem as environmentallyfavorable insofar as coastal environments are increasingly subjected tocorrosive and environmentally harmful “acidified” waters (Feely et al.,Global Biogeochemical Cycles 26 (3) (2012), the disclosure of which isincorporated herein by reference). There are documented impacts oncoastal aquaculture that the addition of alkaline waters could mitigate.

A freshwater reactor is also feasibly operational and the effluentdischarge alkalinity would be controlled by the flow rate and reactionrate for dissolution. Creating alkaline freshwater could beenvironmentally and economically favorable for several applications.Alkaline drinking water is marketed as a healthy alternative to standarddrinking water. The product of carbonate dissolution and CO₂sequestration could be a marketable and economically useful consumable.Another important application of alkaline freshwater is for agriculturalpurposes. Highly acidic soils, such as those in portions of theNortheast and Carolinas are not favorable for certain vegetables(asparagus, broccoli, beets, cabbage, carrots, cauliflower, lettuce,onions, peas, peppers, and spinach). Thus, the use of irrigation waterwith some alkalinity would be preferable treatment or amendment toacidic soils. Coupling a CO₂ sequestration plant to a farming irrigationsystem would yield a doubly beneficial outcome.

DOCTRINE OF EQUIVALENTS

Having described several embodiments, it will be recognized by thoseskilled in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Additionally, a number of well-known processesand elements have not been described in order to avoid unnecessarilyobscuring the present invention. Accordingly, the above descriptionshould not be taken as limiting the scope of the invention.

Those skilled in the art will appreciate that the presently disclosedembodiments teach by way of example and not by limitation. Therefore,the matter contained in the above description or shown in theaccompanying drawings should be interpreted as illustrative and not in alimiting sense. The following claims are intended to cover all genericand specific features described herein, as well as all statements of thescope of the present method and system, which, as a matter of language,might be said to fall there between.

What is claimed is:
 1. A method for carbon dioxide sequestration comprising: dissolving carbon dioxide into an aqueous solution to form an aqueous carbon dioxide solution defined by a mineral undersaturation level; combining the aqueous carbon dioxide solution with a sequestration agent; titrating a hydrating catalyst into the aqueous carbon dioxide solution such that at least within a first catalysis region a mixture of catalyst and aqueous carbon dioxide solution is formed, said first catalysis region encompassing a second interfacial catalysis region located within the laminar boundary layer at the interface between the mixture and the carbonate sequestration agent; and reacting the aqueous carbon dioxide solution with the catalyst within the first catalysis region to produce protons in proximity to the second interfacial catalysis region such that the protons dissolve the sequestration agent; reacting the carbon dioxide within the aqueous carbon dioxide solution with the dissolved sequestration agent within the second interfacial catalysis region to produce an effluent comprising at least bicarbonate; and wherein within the second interfacial catalysis region the dissolution of the sequestration agent is enhanced such that the overall rate of dissolution of the sequestration agent within the catalytic region is higher than the rate of the uncatalyzed dissolution of the sequestration agent when exposed to an aqueous carbon dioxide solution having the same mineral undersaturation level.
 2. The method of claim 1, wherein the sequestration agent is selected from the group consisting of a metal carbonate, or a silicate mineral.
 3. The method of claim 1, wherein the sequestration agent is calcium carbonate and the catalyst is one of either carbonic anhydrase or a carbonic anhydrase analog.
 4. The method of claim 1, wherein the overall rate of dissolution of the carbonate sequestration agent is at least an order of magnitude higher than the rate of the uncatalyzed dissolution of the carbonate sequestration agent when exposed to an aqueous carbon dioxide solution having the same mineral undersaturation level.
 5. The method of claim 1, wherein the mineral undersaturation level is held at less than 0.5.
 6. The method of claim 1, further comprising placing at least the first catalysis region and the second interfacial catalysis region under a pressure of at least 500 psi such that the dissolution of the sequestration agent is increased relative to the unpressurized dissolution rate of the sequestration agent at the same mineral undersaturation.
 7. The method of claim 1, further comprising maintaining at least the second interfacial catalysis region at a temperature no greater than 200° C.
 8. The method of claim 1, further comprising reacting with a condition agent the aqueous solution to reduce surface poisoning ions in the aqueous carbon dioxide solution.
 9. The method of claim 1, wherein the aqueous solution has a circum-neutral pH.
 10. The method of claim 1, wherein the aqueous solution is a brine solution.
 11. The method of claim 1, wherein the aqueous carbon dioxide solution is combined in measured aliquots such that the mineral undersaturation level is maintained at a constant level.
 12. The method of claim 1, further comprising stirring the aqueous solution within at least the first catalysis region such that a mixing zone forms wherein the aqueous carbon dioxide solution and catalyst intermingle and wherein the mixing zone is within the first catalysis region.
 13. The method of claim 12, wherein the stirring forms a diffusion boundary layer around the second interfacial catalysis region the diffusion boundary defining a volume around the interfacial region of the sequestration agent on the order of 10 microns.
 14. The method of claim 1, further comprising roughening the surface of the sequestration agent such that the grain size of the sequestration agent is no greater than 100 μm.
 15. The method of claim 1, further comprising collecting and filtering the effluent from the reaction to capture at least one of catalyst or unreacted aqueous carbon dioxide solution; and reintroducing the catalyst and unreacted aqueous carbon dioxide solution into the first catalysis region.
 16. The method of claim 1, wherein the catalyst operates to at least catalyze the protolysis of water in the aqueous solution and hydrate the CO₂ within the solution.
 17. The method of claim 1, wherein the rate of dissolution is diffusion rate limited.
 18. The method of claim 1, wherein at least one of either the pressure is increased or the temperature is decreased to increase mineral undersaturation.
 19. An apparatus for sequestering carbon dioxide, comprising: at least one reactor vessel defining an enclosed volume; at least one source of a catalyst, a sequestration agent, a CO₂ gas, and an aqueous solution; at least one input in fluid communication between the at least one source and the enclosed volume of the at least one reactor vessel; and at least one output in fluid communication with the enclosed volume of the at least one reactor vessel; wherein the at least one input is arranged such that the CO₂ gas and aqueous solution combine to form an aqueous carbon dioxide solution, and wherein the aqueous carbon dioxide solution and catalyst are delivered as a mixture within the enclosed volume of the at least one reactor within a first catalytic region encompassing a second interfacial catalytic region disposed about the sequestration agent and being located within a laminar flow boundary at the interface between the mixture and the carbonate sequestration agent.
 20. The apparatus of claim 19, wherein at least one of the sequestration agent and catalyst is physically confined within the first catalytic region. 